Cranelift: add debug tag infrastructure. (#11768)

* Cranelift: add debug tag infrastructure.

This PR adds *debug tags*, a kind of metadata that can attach to CLIF
instructions and be lowered to VCo

Cranelift: add debug tag infrastructure. (#11768)

* Cranelift: add debug tag infrastructure.

This PR adds *debug tags*, a kind of metadata that can attach to CLIF
instructions and be lowered to VCode instructions and as metadata on
the produced compiled code. It also adds opaque descriptor blobs
carried with stackslots. Together, these two features allow decorating
IR with first-class debug instrumentation that is properly preserved
by the compiler, including across optimizations and
inlining. (Wasmtime's use of these features will come in followup
PRs.)

The key idea of a "debug tag" is to allow the Cranelift embedder to
express whatever information it needs to, in a format that is opaque
to Cranelift itself, except for the parts that need translation during
lowering. In particular, the `DebugTag::StackSlot` variant gets
translated to a physical offset into the stackframe in the compiled
metadata output. So, for example, the embedder can emit a tag
referring to a stackslot, and another describing an offset in that
stackslot.

The debug tags exist as a *sequence* on any given instruction; the
meaning of the sequence is known only to the embedder, *except* that
during inlining, the tags for the inlining call instruction are
prepended to the tags of inlined instructions. In this way, a
canonical use-case of tags as describing original source-language
frames can preserve the source-language view even when multiple
functions are inlined into one.

The descriptor on a stackslot may look a little odd at first, but its
purpose is to allow serializing some description of
stackslot-contained runtime user-program data, in a way that is firmly
attached to the stackslot. In particular, in the face of inlining,
this descriptor is copied into the inlining (parent) function from the
inlined function when the stackslot entity is copied; no other
metadata outside Cranelift needs to track the identity of stackslots
and know about that motion. This fits nicely with the ability of tags
to refer to stackslots; together, the embedder can annotate
instructions as having certain state in stackslots, and describe the
format of that state per stackslot.

This infrastructure is tested with some compile-tests now;
testing of the interpretation of the metadata output will come with
end-to-end debug instrumentation tests in a followup PR.

* Review feedback: add back sequence points and enforce tags only on sequence points or calls.

* Use Vecs for debug metadata in MachBuffer to avoid SmallVec size penalty in not-used case.

* Review feedback: switch from inlined stackslot descriptor blobs to u64 keys.

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