[HLSL] Update support for user-defined operators to match DXC

Update support for user-defined operators to match DXC. This includes custom operators on structs or classes, or global-scope operators. In general, any operator except assignments should be allowed. Custom assignment operators should report error.

```
struct P {
int p;

float operator=(float f) {
p = (int)f;
}

bool operator<=(float f) {
return p <= (int)f;
}

float operator+=(float f) {
return p += (int)f;
}
};

float operator=(int i) {
return (float)i;
}
```
https://godbolt.org/