[XCOFF][AIX] Fix getSymbol to return the correct qualname when necessary

Summary:
AIX symbol have qualname and unqualified name. The stock getSymbol
could only return unqualified name, which leads us to patch many
caller side(lowerConstant, getMCSymbolForTOCPseudoMO).
So we should try to address this problem in the callee
side(getSymbol) and clean up the caller side instead.

Note: this is a "mostly" NFC patch, with a fix for the original
lowerConstant behavior.

Differential Revision: https://reviews.llvm.org/D78045