[clang] [PowerPC] Add an alias for -mregnames so that full register names used in assembly. (PR #70255)

[Nick Desaulniers via cfe-commits]

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+// REQUIRES: powerpc-registered-target
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nickdesaulniers wrote:

Yeah, a full on integration test for this is a bit much.  The backend tests LGTM; just making sure the frontend doesn't error when presented with this flag (and the `-no` variant) would be sufficient IMO.

https://github.com/llvm/llvm-project/pull/70255