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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