Added QualType::ReadBackpatch to allow QualType initialization with

backpatching. This original was available, but then we removed it. It is back
again to help with deserialization of FieldDecls. Because FieldDecls are
currently owned by RecordDecls, which are owned by a TagType, the type of the
FieldDecl may not be deserialized prior to deserializing the FieldDecl. Thus
backpatching solves the problem of constructing a FieldDecl that references a
type that has not yet been deserialized.

Simplified serialization of TagType to not require passing in the
SerializedPtrID. Registration of the materialized type object is done after
the CreateImpl method returns (as with other types).

llvm-svn: 44143