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

[Kishan Parmar via cfe-commits]

Long5hot wrote:

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

ATR-PASS-COMPLEX-IN-GPRS was missing in clang as shown in ABI doc : https://example61560.wordpress.com/wp-content/uploads/2016/11/powerpc_abi.pdf

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

@AaronBallman, can you guide me, in which section of ReleaseNotes.rst i should add info about new flag?

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