[ThinLTO] Do metadata linking during batch function importing

Summary:
Since we are currently not doing incremental importing there is
no need to link metadata as a postpass. The module linker will
only link in the imported subroutines due to the functionality
added by r256003.

(Note that the metadata postpass linking functionalitiy is still
used by llvm-link, and may be needed here in the future if a more
incremental strategy is adopted.)

Reviewers: joker.eph

Subscribers: joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D16424

llvm-svn: 258458