Started work on new serialization approach within ASTContext to

serialize Type objects in the order they are serialized in the Types
vector.  We also now rely on the methods within ASTContext to unique
Type objects and handle the actual creation of Type objects (these are
now called by the deserialization code).  This approach solves some
hairy issues with ownership of objects and allows us to naturally
handle recursive types.

llvm-svn: 43787