[PDB] Make the native reader support enumerators.

Previously we would dump the names of enum types, but not their
enumerator values.  This adds support for enumerator values.  In
doing so, we have to introduce a general purpose mechanism for
caching symbol indices of field list members.  Unlike global
types, FieldList members do not have a TypeIndex.  So instead,
we identify them by the pair {TypeIndexOfFieldList, IndexInFieldList}.

llvm-svn: 342415