Fix a crash on some platforms where a dSYM for a system library lists a DW_AT_mips_linkage_name for

a non-mangled function - we pass the non mangled string down through abi::__cxa_demangle and it
crashes.  Usually passing non mangled strings to abi::__cxa_demangle works out fine but not
always, apparently.

llvm-svn: 121834