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

[Yuxuan Chen via llvm-commits]

https://github.com/yuxuanchen1997 approved this pull request.

Generally in favor of this change. Given how "normal" flatten is implemented (https://github.com/llvm/llvm-project/commit/41af7c2fdc8cc), I think this patch provides a good foundation to move this feature into LLVM instead of attributing the calls in clang. 

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