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

[Aaron Ballman via cfe-commits]

AaronBallman 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

I'm sorry but I still don't understand why it's not on by default in GNU mode. If the user is in GNU mode, they presumably are doing so because they want to be GNU-compatible, yes?

> > 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?

I'd add it under `New Compiler Flags`

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