[Intrinsics] Add/move some builtin declarations in intrin.h to get...

[Intrinsics] Add/move some builtin declarations in intrin.h to get ms-intrinsics.c to not issue warnings

ud2 and int2c were missing declarations entirely. And the bitscans were only under x86_64, but they seem to be in BuiltinsARM.def as well and are tested by ms_intrinsics.c

Differential Revision: https://reviews.llvm.org/D48187

llvm-svn: 335259