[Intrinsic] Signed and Unsigned Saturation Subtraction Intirnsics

Add an intrinsic that takes 2 integers and perform saturation subtraction on
them.

This is a part of implementing fixed point arithmetic in clang where some of
the more complex operations will be implemented as intrinsics.

Differential Revision: https://reviews.llvm.org/D53783

llvm-svn: 345512