llvm-lto support for generating combined function indexes

Summary:
This patch adds support to llvm-lto that mirrors the support added by
r249270 to the gold plugin. This enables better testing of combined
index generation for ThinLTO.

Added a new test, and this support will be used in the test in D13515.

Reviewers: joker.eph

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13847

llvm-svn: 250699