[pdb] Merge NamedStreamMapBuilder and NamedStreamMap.

While the builder pattern has proven useful for certain other
larger types, in this case it was hampering the ability to use
the data structure, as for runtime access we need a map that
we can efficiently read from and write to.  So the two are merged
into a single data structure that can efficiently be read to,
written from, deserialized from bytes, and serialized to bytes.

llvm-svn: 292664