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

[Grigory Pastukhov via cfe-commits]

grigorypas wrote:

I understand you prefer call-site annotation and see function-level as less useful. However, function-level is important for our internal use case. I'm committed to adding call-site support in a follow-up PR so we support both. Would you be okay with landing function-level first, then adding call-site?

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