Fix llvm-pdbutil to handle new built-in types

Summary:
The built-in PDB types enum has been extended to include char16_t and char32_t.
llvm-pdbutil was hitting an llvm_unreachable because it didn't know about these
new values.  The new values are not yet in the DIA documentation, but are
listed in the cvconst.h header that comes as part of the DIA SDK.

Reviewers: asmith, zturner, rnk

Subscribers: stella.stamenova, llvm-commits, sanjoy

Differential Revision: https://reviews.llvm.org/D43646

llvm-svn: 325838