Resubmit "Pass down the -flto option to the -cc1 job" (r239481)

Resubmit "Pass down the -flto option to the -cc1 job" (r239481)

The patch is the same except for the addition of a new test for the
issue that required reverting the dependent llvm commit.

--Original Commit Message--

Pass down the -flto option to the -cc1 job, and from there into the
CodeGenOptions and onto the PassManagerBuilder. This enables gating
the new EliminateAvailableExternally module pass on whether we are
preparing for LTO.

If we are preparing for LTO (e.g. a -flto -c compile), the new pass is not
included as we want to preserve available externally functions for possible
link time inlining.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241467 91177308-0d34-0410-b5e6-96231b3b80d8