(NFC) Change SubtargetFeatures::ToggleFeature and

SubtargetFeatures::ApplyFeatureFlag to be static, so that
MCSubtargetInfo doesn't need to instantiate SubtargetFeatures
for nothing. Also change the return type to void, as it
wasn't ever used.

This is a partial commit of http://reviews.llvm.org/D15746

llvm-svn: 256823