[HLSL] Define the `hlsl::contained_type` attribute

[bogner]

We need a type attribute that describes the "contained type" of a buffer or resource type. Specifically, our resource types should attribute the __hlsl_resource_t with the template type of the resource, like so:

template <typename T> struct RWBuffer {
  __hlsl_resource_t [[hlsl::contained_type(T)]] h;
};

[bogner]

Partial implementation here: bogner@fb7a9f7

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Justin Bogner (bogner)

We need a type attribute that describes the "contained type" of a buffer or resource type. Specifically, our resource types should attribute the `__hlsl_resource_t` with the template type of the resource, like so: ```hlsl template <typename T> struct RWBuffer { __hlsl_resource_t [[hlsl::contained_type(T)]] h; }; ```