This change is part of this proposal:
https://discourse.llvm.org/t/rfc-all-the-math-intrinsics/78294
This preliminary work adds the intrinsic to llvm and expands using atan
intrinsic for DXIL backend, since DXIL has no atan2 op.
Part 1 for Implement the atan2 HLSL Function #70096.
(reland #108865 reverted in #109842 due to doc build break)