[ThinLTO] Use bulk importing in llvm-link

Summary:
Use bulk importing so we can avoid the use of post-pass metadata
linking. Cloned the ModuleLazyLoaderCache from the FunctionImport pass
to facilitate this.

Reviewers: joker.eph

Subscribers: dexonsmith, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D18455

llvm-svn: 264326