llvm-undname: Add support for demangling typeinfo names

typeinfo names aren't symbols but string constant contents
stored in compiler-generated typeinfo objects, but llvm-cxxfilt
can demangle these for Itanium names.

In the MSVC ABI, these are just a '.' followed by a mangled
type -- this means they don't start with '?' like all MS-mangled
symbols do.

Differential Revision: https://reviews.llvm.org/D67851

llvm-svn: 372602