[HLSL] CodeGen hlsl cbuffer/tbuffer.

cbuffer A {
  float a;
  float b;
}

will be translated to a global variable.

Something like

struct CB_Ty {
  float a;
  float b;
};

CB_Ty A;

And all use of a and b will be replaced with A.a and A.b.

Only support none-legacy cbuffer layout now.
CodeGen for Resource binding will be in separate patch.
In the separate patch, resource binding will map the resource information to the global variable.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D130131