[ThinLTO] Support for specifying function index from pass manager

Summary:
Add a field on the PassManagerBuilder that clang or gold can use to pass
down a pointer to the function index in memory to use for importing when
the ThinLTO backend is triggered. Add support to supply this to the
function import pass.

Reviewers: joker.eph, dexonsmith

Subscribers: davidxl, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D15024

llvm-svn: 254926