block separate from ASTContext. This block is serialized out AFTER writing out
ASTContext, but deserialized BEFORE reading in ASTContext. This permits the
optimization of the serialization of the IdentifierTable where we only write
out identifiers that are used.

This was needed because TagDecls are owned by Types, and TagDecls contain
identifiers. Thus types need to be written out first to register with the
serializer any identifiers they refer to (and hence need to be serialized out
with IdentifierTable).

llvm-svn: 44125