Remove unnecessary demangling operation (NFC)

This extra call to the demangler doesn't affect the performance of C++
because the result is being cached anyway; but I'm working on a patch
to the Swift branch that uses extra contextual information to provide
a more accurate demangling result. In that case this call would be
extra and unnecessary work.

Differential Revision: https://reviews.llvm.org/D58720

llvm-svn: 355042