[modules] When reading / writing a typedef that is a name for linkage for

another declaration, ensure we actually serialize / deserialize that
declaration.

Before this patch, if another copy of the typedef were merged with the parsed
version, we would emit type information referring to the merged version and
consequently emit nothing about the parsed anonymous struct. This resulted in
us losing information, particularly the visible merged module set for the
parsed definition. Force that information to be emitted and to be loaded when
the typedef is used.

llvm-svn: 293219