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

[Zachary Yedidia via cfe-commits]

zyedidia wrote:

I've addressed the earlier comments, let me know your thoughts on testing or other feedback. If you want to reduce the patch size more, we could remove the helper functions from the `MCLFIRewriter` (`isCall`, `isBranch`, etc...) and just query that information directly in each rewriter.

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