[clang] [llvm] [LFI] Add MCLFIRewriter infrastructure (PR #172906)

[Zachary Yedidia via cfe-commits]

zyedidia wrote:

Okay thank you, sounds good. I have tried to reduce header usage as much as possible, and opened a PR: #182617. Is there any way we can test compile-time impact ahead of merging to confirm the improvement?

I'm also seeing these buildbot CI failures, but from what I can tell they seem unrelated to these changes. Let me know if there is any action to be taken on that front.

https://github.com/llvm/llvm-project/pull/172906