[clang] [clang][PowerPC] Do not extend _BitInt arguments and returns on PPC64 (PR #231734)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> Having said that, Michael Meissner has agreed upon PowerPC64 ABI for _BitInt, though it is yet to be published.

The group that is tasked with updating the ABI document is not tasked with rubber stamping.

https://github.com/llvm/llvm-project/pull/231734