[HLSL] Add `ConstantBuffer<T>` template definition to `HLSLExternalSemaSource` and enable direct member access

[hekota]

Add ConstantBuffer<T> definition to HLSLExternalSemaSource. The resource definition will contain a handle that will be lowered to a dx.CBuffer target extension type.

Note that members of a struct that is passed as a template argument to ConstantBuffer can be accessed directly as if they are members of the ConstantBuffer class.

struct MyConstants {
    int MyConstValue;
};

ConstantBuffer<MyConstants> CB : register(b0);

RWBuffer<int> Buf;

[numthreads(32, 1, 1)]
void main(uint3 dispatch_thread_id : SV_DispatchThreadID) {
    Buf[0] = CB.MyConstValue;
}

https://godbolt.org/z/6GdE383j3

Refer to Proposal for mapping resource attributes for resource attribute mapping.

Depends on llvm/wg-hlsl#178