[DirectX] Start documenting DXIL Resource handling

This adds a new document about DXIL Resource Handling. I've attempted
to describe here how we intend to use TargetExtTypes to represent
resources in LLVM IR and the various intrinsics we'll need to lower
these through LLVM to DXIL.

There are a number of TODOs in the document, but I think it makes
sense to publish as is and evolve it as we start implementing this.

Author: bogner

clang/docs/HLSL/HLSLIRReference.rst

@@ -11,15 +11,6 @@ Introduction
 The goal of this document is to provide a reference for all the special purpose
 IR metadata and attributes used by the HLSL code generation path.
 
-IR Metadata
-===========
-
-``hlsl.uavs``
--------------
-
-The ``hlsl.uavs`` metadata is a list of all the external global variables that
-represent UAV resources.
-
 Function Attributes
 ===================
 

clang/docs/HLSL/ResourceTypes.rst

@@ -29,6 +29,7 @@ pointer of the template parameter type. The pointer is populated from a call to
 data through until lowering in the backend.
 
 Resource types are annotated with the ``HLSLResource`` attribute, which drives
-code generation for resource binding metadata. The ``hlsl`` metadata nodes are
-transformed in the backend to the binding information expected by the target
-runtime.
+code generation into target extension types in IR. These types are target
+specific and differ between DXIL and SPIR-V generation, providing the necessary
+information for the targets to generate binding metadata for their respective
+target runtimes.