[Intrinsic] Unsigned Fixed Point Multiplication Intrinsic

Add an intrinsic that takes 2 unsigned integers with the scale of them
provided as the third argument and performs fixed point multiplication 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/D55625

llvm-svn: 353059