[ARM] Do not test for CPUs, use SubtargetFeatures (Part 3). NFCI

This is a follow-up for r273544 and r273853.

The end goal is to get rid of the isSwift / isCortexXY / isWhatever methods.
This commit also marks them as obsolete.

Differential Revision: http://reviews.llvm.org/D21796

llvm-svn: 274616