PR48434: Work around crashes due to deserialization cycles via typedefs.
Ensure that we can deserialize a TypedefType even while in the middle of
deserializing its TypedefDecl, by removing the need to look at the
TypedefDecl while constructing the TypedefType.
This fixes all the currently-known failures for PR48434, but it's not a
complete fix, because we can still trigger deserialization cycles, which
are not supposed to happen.