[DirectX] Set entry function attribute optnone if optimizations are disabled

[bharadwajy]

dx.disable_optimizations that has been added to HLSL suppport in clang appears to be potentially unnecessary and not defined in DXIL.rst. So, replace it by setting the function attribute Attribute::OptimizeNone on entry functions of the module to indicate presence of -Od flag on the commandline.

Implement this change and add necessary tests.