Make __shiftleft128 / __shiftright128 real compiler built-ins.

r337619 added __shiftleft128 / __shiftright128 as functions in intrin.h.
Microsoft's STL plans on using these functions, and they're using intrin0.h
which just has declarations of built-ins to not pull in the huge intrin.h
header in the standard library headers. That requires that these functions are
real built-ins.

https://reviews.llvm.org/D50907

llvm-svn: 340048