Synchronize LLVM's copy of libc++abi's demangler with the libc++abi
version after r371273.

Also fix a minor issue in r371273 that only surfaced after template
instantiation from LLVM's use of the demangler.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@371274 91177308-0d34-0410-b5e6-96231b3b80d8