[LowerTypeTests] Discard extern_weak linkage for definitions

Summary:
Fix PR37625. It's possible for an extern_weak declaration to be emitted
to the merged module when a definition exists in the ThinLTO portion of
the build; discard the linkage on the declaration in that case.
(otherwise we copy the linkage to the alias to the jumptable and fail)

Reviewers: pcc

Reviewed By: pcc

Subscribers: mehdi_amini, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D47494

llvm-svn: 333604