[HLSL] Add shader attribute

Shader attribute is for shader library identify entry functions.
Here's an example,

[shader("pixel")]
float ps_main() : SV_Target {
  return 1;
}

When compile this shader to library target like -E lib_6_3, compiler needs to know ps_main is an entry function for pixel shader. Shader attribute is to offer the information.

A new attribute HLSLShader is added to support shader attribute. It has an EnumArgument which included all possible shader stages.

Reviewed By: aaron.ballman, MaskRay

Differential Revision: https://reviews.llvm.org/D123907