[PDB] Refactor a little of the Symbol creation code.

Eventually we need to be able to support nested types, which don't
have an associated CVType record.  To handle this, remove the
CVType from all of the record classes, and instead store the
deserialized record.  Then move the deserialization up to the thing
that creates the type.  This actually makes error handling better
anyway as we can return an invalid symbol instead of asserting false.

llvm-svn: 342284