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

llvm-svn: 229831