[HLSL] implement `atan` intrinsic

[llvm-beanz]

Implement HLSL atan intrinsic:

https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-atan

part of this proposal: https://discourse.llvm.org/t/rfc-all-the-math-intrinsics/78294

Four parts:

1. DXIL backend change
2. SPIRV backend change
3. HLSL frontend change
4. Constraint intrinsic change