Implement MS _BitScan intrinsics

Summary: _BitScan intrinsics (and some others, for example _Interlocked and _bittest) are supposed to work on both ARM and x86. This is an attempt to isolate them, avoiding repeating their code or writing separate function for each builtin.

Reviewers: hans, thakis, rnk, majnemer

Subscribers: RKSimon, cfe-commits, aemerson

Differential Revision: https://reviews.llvm.org/D25264

llvm-svn: 284060