[clang] [Doc][HLSL] Add documentation for root signature. (PR #83933)

[Chris B via cfe-commits]

https://github.com/llvm-beanz commented:

It might be useful to have a section in this document that briefly describes what is done at each phase of the compiler and what information is passed through to the next.

For example:
ClangSema parses the attribute and generates <X>. ClangCodeGen reads <X> and generates <Y>. <Pass YtoZ> in the DirectX backend transforms/validates <Y> yo <Z>. LLVMMC emits <Z> to <Output>.

Also we need to keep in mind that some of these structures need to be available in the LLVM Binary Format and Object libraries so that the object parser can read root signatures. We should think about what bits of code need to live where to maintain the correct layers in the compiler architecture.

https://github.com/llvm/llvm-project/pull/83933