add floor library function

This change exposes the floor library function for HLSL,
excluding long, int, and long long doubles.
Floor is supported for all scalar, vector, and matrix types.

Long and long long double support is missing in this patch because those types
don't exist in HLSL. Int is missing because the floor function only works on floating type arguments.

The full documentation of the HLSL floor function is available here:
https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-floor

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D139137