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.

Differential Revision: http://reviews.llvm.org/D7065

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229831 91177308-0d34-0410-b5e6-96231b3b80d8