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