[ThinLTO] Option to invoke ThinLTO backend passes and importing

Description

[ThinLTO] Option to invoke ThinLTO backend passes and importing

Summary:
Adds new option -fthinlto-index=<file> to invoke the LTO pipeline
along with function importing via clang using the supplied function
summary index file. This supports invoking the parallel ThinLTO
backend processes in a distributed build environment via clang.

Additionally, this causes the module linker to be invoked on the bitcode
file being compiled to perform any necessary promotion and renaming of
locals that are exported via the function summary index file.

Add a couple tests that confirm we get expected errors when we try to
use the new option on a file that isn't bitcode, or specify an invalid
index file. The tests also confirm that we trigger the expected function
import pass.

Depends on D15024

Reviewers: joker.eph, dexonsmith

Subscribers: joker.eph, davidxl, cfe-commits

Differential Revision: http://reviews.llvm.org/D15025