DebugInfo: Sink the code emitting DW_AT_APPLE_omit_frame_ptr down to a more common spot.

No functional change. Pre-emptive refactoring before I start pushing
some of this subprogram creation down into DWARFCompileUnit so I can
build different subprograms in the skeleton unit from the dwo unit for
adding -gmlt-like data to the skeleton.

llvm-svn: 218713