[clang] [HLSL][Docs] Add documentation for HLSL functions (PR #75397)

[Chris B via cfe-commits]

llvm-beanz wrote:

@rjmccall, I'm curious if you have any thoughts on the proposed implementation approach here?

The TL;DR for the gnarly bit is to have AST nodes representing parameters that need temporary values, and for "output" parameters where there may be cast sequences involved the AST node will capture the cast sequences for both parameter initialization and writing back to the argument lvalue. Then we can slightly tweak the CGCall write back support to support having a cast sequence in the AST.

https://github.com/llvm/llvm-project/pull/75397