Fix logic around determining use of frame pointer with -pg.
As part of r342165, I rewrote the logic to check whether
-fno-omit-frame-pointer was passed after a -fomit-frame-pointer
argument. This CL switches that logic to use the consolidated
shouldUseFramePointer() function. This fixes a potential issue where -pg
gets used with -fomit-frame-pointer on a platform that must always retain
Reviewed By: dblaikie
Differential Revision: https://reviews.llvm.org/D52191