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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> There are still broken bots from this change: [lab.llvm.org/buildbot/#/builders/124/builds/8595](https://lab.llvm.org/buildbot/#/builders/124/builds/8595) @zjaffal, please revert the changes until you have a fix ready.

The GCC 7.5 build issue was due to `return Filter;`. I fixed it and the reserved identifier `_GroupBy` (introduced by a follow-up c136e722aa4b03209da48b641c6f413202cb0ff9 to this patch) in 0603737ac0f73ff33326d0274b3152cc5a81e1ec.

It would be cleaner the revert the patch, but there are so many follow-up commits (and would cause conflicts), so I added another follow-up instead of reverting the whole patch.

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