[HLSL] Layout compatible type casting

[llvm-beanz]

HLSL allows explicit casting between layout compatible types. For example:

struct XYZ {
  float X, Y, Z;
};

XYZ xyz = ...;
float3 f3 = (float3)xyz;
xyz = (XYZ)f3;

struct S {
int i;
float f;
};

struct OtherS {
int i;
float f;
};

S s =...;
OtherS o = (OtherS)s;

Related: #100614