TableGen: introduce support for MSBuiltin

Add MSBuiltin which is similar in vein to GCCBuiltin.  This allows for adding
intrinsics for Microsoft compatibility to individual instructions.  This is
needed to permit the creation of ARM specific MSVC extensions.

This is not currently in use, and requires an associated change in clang to
enable use of the intrinsics defined by this new class.  This merely sets the
LLVM portion of the infrastructure in place to permit the use of this
functionality.  A separate set of changes will enable the new intrinsics.