[ThinLTO] Support for specifying function index from pass manager

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254926 91177308-0d34-0410-b5e6-96231b3b80d8