[llvm-remarkutil] Add an option to print out function sizes

[llvm-remarkutil] Add an option to print out function sizes

This adds an `instruction-count` command to llvm-remarkutil.

```
llvm-remarkutil instruction-count --parser=<bitstream|yaml> <file>
```

This will, for now, only print out asm-printer `InstructionCount` remarks.

Frequently I need to find out things like "what are the top 10 largest
functions" in a given project.

This makes it so we can find that information quickly and easily from any
format of remarks.

I chose a CSV because I usually want to stick these into a spreadsheet, and
the data is two-dimensional.

In the future, we may want to change this to another format if we add more
complicated data.

Differential Revision: https://reviews.llvm.org/D134765