[clang] [HLSL][docs] Document hlsl.h in the HLSL docs (PR #84081)

[S. Bharadwaj Yadavalli via cfe-commits]

================
@@ -114,6 +114,44 @@ not re-targetable, we want to share the Clang CodeGen implementation for HLSL
 with other GPU graphics targets like SPIR-V and possibly other GPU and even CPU
 targets.
 
+hlsl.h
+------
+
+HLSL has an extensive library of functionality. This is similar to OpenCL and
+CUDA. The implementation approach for the HLSL library functionality draws from
+patterns in use by OpenCL and other Clang resource headers.
+
+Similar to OpenCL, the HLSL library functionality is implicitly declared in
+translation units without needing to include a header to provide declarations.
+In Clang this is handled by making ``hlsl.h`` an implicitly included header
+distributed as part of the Clang resource directory.
+
+Similar to OpenCL, HLSL's implicit header will explicitly declare all overloads,
+and each overload will map to a corresponding ``__builtin_*`` compiler intrinsic
+that is handled in ClangCodeGen. CUDA uses a similar pattern although many CUDA
+functions have full definitions in the included headers which in turn call
+corresponding ``__builtin*`` compiler intrinsics. By not having bodies HLSL
+avoids the need for the inliner to clean up and inline large numbers of small
+library functions.
+
+HLSL's implicit headers also define some of HLSL's typedefs. This is consistent
+with how the AVX vector header is implemented.
+
+Concerns have been expressed that this approach may result in slower compile
+times than the approach DXC uses where library functions are treated more like
+Clang ``__builtin*`` intrinsics. No real world use cases have been identified
+where parsing is a significant compile-time overhead, but the HLSL implicit
+headers can be compiled into a module for performance if needed.
+
+Further, by treating these functions as functions rather than ``__builtin*``
+compiler intrinsics the language behaviors are more consistent and aligned with
----------------
bharadwajy wrote:

```suggestion
compiler intrinsics, the language behaviors are more consistent and aligned with
```

https://github.com/llvm/llvm-project/pull/84081