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

[Fangrui Song via llvm-commits]

================
@@ -4798,6 +4798,10 @@ def mrop_protect : Flag<["-"], "mrop-protect">,
     Group<m_ppc_Features_Group>;
 def mprivileged : Flag<["-"], "mprivileged">,
     Group<m_ppc_Features_Group>;
+def mregnames : Flag<["-"], "mregnames">, Group<m_ppc_Features_Group>,
+                Visibility<[ClangOption]>;
----------------
MaskRay wrote:

`Visibility<[ClangOption]>` is the default and should be omitted.

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