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,
       <c...