[clang] Remove duplication in types::getCompilationPhases()

Call Driver::getFinalPhase() instead of duplicating it.

https://reviews.llvm.org/D65993 added the duplication, then
02e35832 maded it more obviously a copy of getFinalPhase().

The only difference is that getCompilationPhases() used to use
LastPhase / IfsMerge where getFinalPhase() used Link. Adapt
getFinalPhase() to return IfsMerge when needed.

No intentional behavior change.

Differential Revision: https://reviews.llvm.org/D110770