[clang] [llvm] [clang/LLVM] Add flatten_deep attribute for depth-limited inlining (1/2) (PR #165777)

[Aiden Grossman via cfe-commits]

https://github.com/boomanaiden154 commented:

What exactly is the motivation for this?

This also kind of changes the semantics of `alwaysinline`, which states the functions should be inlined always unless illegal. I guess this can be viewed as making inlining illegal in such circumstances, but a LangRef update would probably be good either way.

https://github.com/llvm/llvm-project/pull/165777