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.

llvm-svn: 371274
GitOrigin-RevId: 0e881b9f5d1fd3c615f79860eacedaecad03daea
1 file changed