Add JSON output option to llvm-remark-size-diff

This adds JSON output to llvm-remark-size-diff.

The goal here is to make it easy for external tools to consume output from
llvm-remark-size-diff. The

Add JSON output option to llvm-remark-size-diff

This adds JSON output to llvm-remark-size-diff.

The goal here is to make it easy for external tools to consume output from
llvm-remark-size-diff. These tools could be used for automated size analysis.
(E.g. in CI).

To specify JSON output, use `--report_style=json`. JSON output can be
pretty-printed via `--pretty`.

With automation in mind, the schema looks like this:

```
"Files": {
"A": <filename_a>
"B": <filename_b>
},

"InBoth": [
{
"FunctionName": <function name>,
"InstCount": [
<count_in_a>,
<count_in_b>
],
"StackSize": [
<count_in_a>,
<count_in_b>
]
},
...
]

"OnlyInA": [
{
"FunctionName": <function name>,
"InstCount": [
<count_in_a>,
0
],
"StackSize": [
<count_in_a>,
0
]
},
...
]

"OnlyInB": [
{
"FunctionName": <function name>,
"InstCount": [
0,
<count_in_b>
],
"StackSize": [
0,
<count_in_b>
]
},
...
]
```

A few notes:

- Filenames are included, because tools may want to combine many outputs
together in some way (a big JSON file, a big CSV, or something.)

- Counts are represented as [a, b] so that a diff can be calculated via b - a.
The original counts may be useful for size analysis (e.g. was this function
extremely large before?) and so both are preserved.

- `OnlyInA` and `OnlyInB` have a 0 for one of the counts always. This is to
make it easier for tools to share code between `OnlyInA`, `OnlyInB`, and
`InBoth`.

Differential Revision: https://reviews.llvm.org/D121173

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Fix Wdocumentation unknown parameter warning

[llvm-remark-size-diff] Don't use enum name as auto variable name

This was confusing the clang-cmake-x86_64-avx2-linux buildbot (gcc version 5.4.0).

Fix buildbot failure after D112940

Change a type from `int64_t` to `long long` to make a buildbot happy.

https://lab.llvm.org/buildbot/#/builders/196/builds/7371

```
error: no matching function fo

Fix buildbot failure after D112940

Change a type from `int64_t` to `long long` to make a buildbot happy.

https://lab.llvm.org/buildbot/#/builders/196/builds/7371

```
error: no matching function for call to 'getAsSignedInteger'
if (getAsSignedInteger(ValStr, 0, Val))
```

Worked With My Compiler (TM)

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Add a tool for diffing size remarks

This is a tool which can handle bitstream and YAML remarks. The idea here is to
provide more insight into which functions changed in a benchmark when testing
comp

Add a tool for diffing size remarks

This is a tool which can handle bitstream and YAML remarks. The idea here is to
provide more insight into which functions changed in a benchmark when testing
compiler changes.

E.g. "foo got 20% bigger, so maybe we should look more closely at that."

To use the tool, you can use...

```
$ llvm-remark-size-diff remarks_file_a remarks_file_b --parser=yaml|bitstream
```

... on two remarks files containing at least instruction count remarks. This
will output some data on instruction count change and also other relevant
information such as stack size change from `remarks_file_a` to `remarks_file_b`.

This is a bit of a WIP so I'm happy to change the format etc. Ultimately I think
it'd be best to have some JSON output which could be consumed by another tool.
But some base-level, greppable output is very handy to have anyway.

The format I'm proposing here is

```
<files> <inc/dec in inst count> <fn name> <inst count change> <stack B change>
```

Where the files and increase/decrease are indicated like below:

- `<files>` is one of `++` (file B), `--` (file A), `==` (both)
- `<inc/dec in inst count>` is one of `>` (increase) or `<` (decrease)

This makes it easy to grep for things like "which functions appeared in A but
did not appear in B?" Or "what are all the instruction count decreases?"

Differential Revision: https://reviews.llvm.org/D112940

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