[clang] [clang][PowerPC] Add flag to enable compatibility with GNU for complex arguments (PR #77732)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I'm wondering why the GNU-compatible behavior is not on by default, especially when in GNU mode (e.g., `-std=gnu17`)?

Also, the changes should come with a release note in `clang/docs/ReleaseNotes.rst` so users know about the new functionality.

https://github.com/llvm/llvm-project/pull/77732