Use std::bitset for SubtargetFeatures

Use std::bitset for SubtargetFeatures

Previously, subtarget features were a bitfield with the underlying type being uint64_t.
Since several targets (X86 and ARM, in particular) have hit or were very close to hitting this bound, switching the features to use a bitset.
No functional change.

The first two times this was committed (r229831, r233055), it caused several buildbot failures.
At least some of the ARM and MIPS ones were due to gcc/binutils issues, and should now be fixed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237234 91177308-0d34-0410-b5e6-96231b3b80d8