Move LinkerInput to LinkingContext.

Summary:
The aim of doing this is to make LinkerInput accessible from outside of
Driver::link(). This should be useful for COFF, which allows object files
to add more input files by linker options in .drectve section.

Reviewers: kledzik, shankarke

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1569