[ARM][TargetParser] Improve handling of dependencies between target features

The patch at https://reviews.llvm.org/D64048 added "negative"
dependency handling in `ARM::appendArchExtFeatures`: feature "noX"
removes all features, which imply "X".

This patch adds the "positive" handling: feature "X" adds all the
feature strings implied by "X".

(This patch also comes from the suggestion here
https://reviews.llvm.org/D72633#inline-658582)

Differential Revision: https://reviews.llvm.org/D72762