[PDB] Introduce `MSVCUndecoratedNameParser`

This patch introduces the simple MSVCUndecoratedNameParser. It is needed for
parsing names of PDB symbols corresponding to template instantiations. For
example, for the name `operator<<A>'::`2'::B::operator> we can't just split the
name with :: (as it is implemented for now) to retrieve its scopes. This parser
processes such names in a more correct way.

Differential Revision: https://reviews.llvm.org/D52461

llvm-svn: 346213