[HLSL] Design of constructors for resource handle initialization

[hekota]

This needs design / planning work before we can estimate it.

Resource handle initialization should be done via resource class constructors. If a resource is declared in a global variable, the compiler should make sure to call a constructor that takes binding information, which will later be translated to dx.handle.fromBinding.

Resources declared as local variables would use a default constructor that will initialize the handle to 'empty' or 'null', or simply leave it undefined. Another resource can be assigned to these local variables and a copy constructor and/or assignment operator will take care of copying the resource handle.

This should greatly simplify handling of resources, including when they are dynamically bound, used as function in/out parameters, declared in unbounded arrays or used defined types.

Resource classes should look like this (although at this point we have to implement it in HLSLExternalSemaSource since it uses features HLSL doesn't have yet):

struct Binding;

template <typename T> struct RWBuffer {
private:
  using handle_t = __hlsl_resource_t
      [[hlsl::contained_type(T)]] [[hlsl::resource_class(UAV)]];
  handle_t h;

public:
  // For uninitialized handles
  RWBuffer() { h = __builtin_hlsl_create_poison_handle(); }
  
  // Resources are copyable.
  RWBuffer(RWBuffer &LHS) = default;
  
  // For handles constructed as globals with binding information, or from the
  // descriptor heap.
  RWBuffer(Binding &B) { h = __builtin_create_handle_from_binding(B); }

  ~RWBuffer() = default;

  RWBuffer &operator=(RWBuffer &LHS) {
    h = LHS.h;
    return *this
  }
...  
}

Follow-up from llvm/llvm-project#105076.

Open Questions:

• What does __builtin_hlsl_create_poison_handle() actually do?
• How does struct Binding work? Maybe constructor should just take register info.
• How will this work for SPIR-V?
• How does this interact with Support arrays of resources #127 and Document support of resources inside user-defined structs #175

[llvm-beanz]

We've also defined inout parameters as initializing in with the copy constructor and assigning back with the assignment operator, so having those both defined for resources will make everything work as expected. They can both be = default.