[MC] Fix some Clang-tidy modernize and Include What You Use warnings in SubtargetFeature; other minor fixes (NFC).

Same changes in files affected by reduced SubtargetFeature.h dependencies.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294548 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed