[llvm] [Remarks] Introduce tool to count remarks. (PR #66214)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

My LLVM build is broken with this commit:

```
In file included from /user/rarm/src/riscv-sdk-llvm/llvm/tools/llvm-remarkutil/RemarkCounter.cpp:13:
/user/rarm/src/riscv-sdk-llvm/llvm/tools/llvm-remarkutil/RemarkCounter.h:113:11: error: declaration of ‘llvm::remarks::GroupBy llvm::remarks::Counter::GroupBy’ changes meaning of ‘GroupBy’ [-fpermissive]
  113 |   GroupBy GroupBy;
      |           ^~~~~~~
```

https://github.com/llvm/llvm-project/pull/66214