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

[Zachary Yedidia via cfe-commits]

zyedidia wrote:

Sounds good, thank you! Also pinging @MaskRay in case you'd like to weigh in. This is the change that will unlock our ability to move forward with the AArch64 LFI rewriter.

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