Enable llvm-pdbutil to list enumerations using native PDB reader

This extends the native reader to enable llvm-pdbutil to list the enums in a
PDB and it includes a simple test. It does not yet list the values in the
enumerations, which requires an actual implementation of
NativeEnumSymbol::FindChildren.

To exercise this code, use a command like:

    llvm-pdbutil pretty -native -enums foo.pdb

Differential Revision: https://reviews.llvm.org/D35738

llvm-svn: 310144