[clang] [llvm] [clang/LLVM] Add function level flatten_depth attribute for depth-limited inlining (PR #165777)

[Andrew Pinski via llvm-commits]

pinskia wrote:

> Thank you for your feedback and for raising these concerns. To clarify, our primary use case at Meta is to completely flatten functions by inlining the entire call tree. The max depth parameter is not intended as a core part of the user workflow, but rather as a safeguard to prevent issues if the call tree happens to be extremely deep.

See https://github.com/llvm/llvm-project/issues/60689 which says the current LLVM's implementation of flatten is different from GCC's.

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