[ThinLTO] Only compute imports for current module in FunctionImport pass

Summary:
The function import pass was computing all the imports for all the
modules in the index, and only using the imports for the current module.
Change this to instead compute only for the given module. This means
that the exports list can't be populated, but they weren't being used
anyway.

Longer term, the linker can collect all the imports and export lists
and serialize them out for consumption by the distributed backend
processes which use this pass.

Reviewers: joker.eph

Subscribers: llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D18945

llvm-svn: 266125