[Intrinsic] Signed Fixed Point Multiplication Intrinsic

[Intrinsic] Signed Fixed Point Multiplication Intrinsic

Add an intrinsic that takes 2 signed 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/D54719

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348912 91177308-0d34-0410-b5e6-96231b3b80d8