[ThinLTO] Appending linkage fixes

Summary:
Fix import from module with appending var, which cannot be imported. The
first fix is to remove an overly-aggressive error check.

The second fix is to deal with restructuring introduced to the module
linker yesterday in r254418 (actually, this fix was included already
in r254559, just added some additional cleanup).

Test by Mehdi Amini.

Reviewers: joker.eph, rafael

Subscribers: joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D15156

llvm-svn: 254624