Revert r286788

The Itanium ABI [1] specifies that __cxa_demangle accept either:

   1) symbol names, which start with "_Z"
   2) type manglings, which do not start with "_Z"

r286788 erroneously assumes that it should only handle symbols, so this patch
reverts it and adds a counterexample to the testcase.

1: https://mentorembedded.github.io/cxx-abi/abi.html#demangler


Reviewers: zygoloid, EricWF
llvm-svn: 292418