Design and document unorm/snorm functionality

[pow2clk]

Is your feature request related to a problem? Please describe.

There are a number of issues that have cropped up related to unorm/snorm in DXC that we want to avoid by better design in the future.
 

• Emit an error/warning on incorrect uses of snorm/unorm DirectXShaderCompiler#2264
  • related: ByteAddressBuffer.Load<T> with normalized signed short DirectXShaderCompiler#2260
• DXC allows assigning a Buffer<snorm float> to a Buffer<unorm float> DirectXShaderCompiler#2106
• DXC ignores unorm/snorm on [RW]Buffer/[RW]Texture** element type DirectXShaderCompiler#1722 (incompletely fixed)

Describe the solution you'd like
A specification that defines all the ways unorm and snorm can and can't be used and how they might behave differently from other float types. In particular how assignments between them can work and how they can be used with resources are areas that many of the above issues touch on.

Describe alternatives you've considered
I think it's important to note the pitfalls that have occurred before and have a task to address the issue at large to ensure it doesn't get overlooked rather than expecting we'll do it right when the time comes.