Pass down the -flto option to the -cc1 job, and from there into the

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@239481 91177308-0d34-0410-b5e6-96231b3b80d8