[clang][NFC] Consolidating usage of "FinalPhase" in Driver::BuildActions.

I am working to remove this concept of the "FinalPhase" in the clang driver,
but it is used in a lot of different places to do argument handling for
different combinations of phase pipelines and arguments. I am trying to
consolidate most of the uses of "FinalPhase" into its own separate scope.
Eventually, in a subsequent patch I will move all of this stuff to a separate
function, and have more of the complication phase list construction setup into
types::getComplicationPhases.

Differential Revision: https://reviews.llvm.org/D65969

llvm-svn: 368393