[PDLL] Add support for user defined constraint and rewrite functions

These functions allow for defining pattern fragments usable within the `match` and `rewrite` sections of a pattern. The main stru

[PDLL] Add support for user defined constraint and rewrite functions

These functions allow for defining pattern fragments usable within the `match` and `rewrite` sections of a pattern. The main structure of Constraints and Rewrites functions are the same, and are similar to functions in other languages; they contain a signature (i.e. name, argument list, result list) and a body:

```pdll
// Constraint that takes a value as an input, and produces a value:
Constraint Cst(arg: Value) -> Value { ... }

// Constraint that returns multiple values:
Constraint Cst() -> (result1: Value, result2: ValueRange);
```

When returning multiple results, each result can be optionally be named (the result of a Constraint/Rewrite in the case of multiple results is a tuple).

These body of a Constraint/Rewrite functions can be specified in several ways:

* Externally
In this case we are importing an external function (registered by the user outside of PDLL):

```pdll
Constraint Foo(op: Op);
Rewrite Bar();
```

* In PDLL (using PDLL constructs)
In this case, the body is defined using PDLL constructs:

```pdll
Rewrite BuildFooOp() {
// The result type of the Rewrite is inferred from the return.
return op<my_dialect.foo>;
}
// Constraints/Rewrites can also implement a lambda/expression
// body for simple one line bodies.
Rewrite BuildFooOp() => op<my_dialect.foo>;
```

* In PDLL (using a native/C++ code block)
In this case the body is specified using a C++(or potentially other language at some point) code block. When building PDLL in AOT mode this will generate a native constraint/rewrite and register it with the PDL bytecode.

```pdll
Rewrite BuildFooOp() -> Op<my_dialect.foo> [{
return rewriter.create<my_dialect::FooOp>(...);
}];
```

Differential Revision: https://reviews.llvm.org/D115836

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[PDLL] Add a `rewrite` statement to enable complex rewrites

The `rewrite` statement allows for rewriting a given root
operation with a block of nested rewriters. The root operation is
not implicitly

[PDLL] Add a `rewrite` statement to enable complex rewrites

The `rewrite` statement allows for rewriting a given root
operation with a block of nested rewriters. The root operation is
not implicitly erased or replaced, and any transformations to it
must be expressed within the nested rewrite block. The inner body
may contain any number of other rewrite statements, variables, or
expressions.

Differential Revision: https://reviews.llvm.org/D115299

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[PDLL] Add a `replace` rewrite statement for replacing operations

This statement acts as a companion to the existing `erase`
statement, and is the corresponding PDLL construct for the
`PatternRewrit

[PDLL] Add a `replace` rewrite statement for replacing operations

This statement acts as a companion to the existing `erase`
statement, and is the corresponding PDLL construct for the
`PatternRewriter::replaceOp` C++ API. This statement replaces a
given operation with a set of values.

Differential Revision: https://reviews.llvm.org/D115298

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[PDLL] Add support for tuple types and expressions

Tuples are used to group multiple elements into a single
compound value. The values in a tuple can be of any type, and
do not need to be of the sam

[PDLL] Add support for tuple types and expressions

Tuples are used to group multiple elements into a single
compound value. The values in a tuple can be of any type, and
do not need to be of the same type. There is also no limit to
the number of elements held by a tuple.

Tuples will be used to support multiple results from
Constraints and Rewrites (added in a followup), and will also
make it easier to support more complex primitives (such as
range based maps that can operate on multiple values).

Differential Revision: https://reviews.llvm.org/D115297

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[PDLL] Add support for `op` Operation expressions

An operation expression in PDLL represents an MLIR operation. In
the match section of a pattern, this expression models one of
the input operations

[PDLL] Add support for `op` Operation expressions

An operation expression in PDLL represents an MLIR operation. In
the match section of a pattern, this expression models one of
the input operations to the pattern. In the rewrite section of
a pattern, this expression models one of the operations to
create. The general structure of the operation expression is very
similar to that of the "generic form" of textual MLIR assembly:

```
let root = op<my_dialect.foo>(operands: ValueRange) {attr = attr: Attr} -> (resultTypes: TypeRange);
```

For now we only model the components that are within PDL, as PDL
gains support for blocks and regions so will this expression.

Differential Revision: https://reviews.llvm.org/D115296

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[PDLL] Add support for literal Attribute and Type expressions

This allows for using literal attributes and types within PDLL,
which simplifies building both constraints and rewriters. For
example, c

[PDLL] Add support for literal Attribute and Type expressions

This allows for using literal attributes and types within PDLL,
which simplifies building both constraints and rewriters. For
example, checking if an attribute is true is as simple as
`attr<"true">`.

Differential Revision: https://reviews.llvm.org/D115295

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[mlir][PDLL] Add an initial frontend for PDLL

This is a new pattern rewrite frontend designed from the ground
up to support MLIR constructs, and to target PDL. This frontend
language was proposed in

[mlir][PDLL] Add an initial frontend for PDLL

This is a new pattern rewrite frontend designed from the ground
up to support MLIR constructs, and to target PDL. This frontend
language was proposed in https://llvm.discourse.group/t/rfc-pdll-a-new-declarative-rewrite-frontend-for-mlir/4798

This commit starts sketching out the base structure of the
frontend, and is intended to be a minimal starting point for
building up the language. It essentially contains support for
defining a pattern, variables, and erasing an operation. The
features mentioned in the proposal RFC (including IDE support)
will be added incrementally in followup commits.

I intend to upstream the documentation for the language in a
followup when a bit more of the pieces have been landed.

Differential Revision: https://reviews.llvm.org/D115093

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