[HLSL] Add diagnostics for Buffers/RWBuffers with invalid element types

[bogner]

There's a specific closed set of types that are valid as element types for TypedBuffer types, which includes int/uint of sizes 16/32/64, half, float, and double. If someone writes RWBuffer<MyCustomType> we should reject it and give the user a nice diagnostic.

AC:

• A spec is created to dictate the validation behavior
• Type validation happens prior to code generation
• Tests written to validate failure on mixed types

[davidcook-msft]

Moved to low as this currently does not happen and the DXIL validator will catch it, but we would rather catch it prior to the validator

[bogner]

We could arguably implement this simply by decorating the resource classes with an appropriate concept (something along the lines of https://godbolt.org/z/Y3sezEe36)