[DirectX] Start documenting DXIL Resource handling (#90553)

Summary:
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.

For now this document is limited to the high level concepts and a few
details on buffer types, and there are a number of TODOs in the document
that we'll iterate on as we progress in the implementation.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251613

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

@@ -19,16 +19,15 @@ In Clang resource types are forward declared by the ``HLSLExternalSemaSource``
 on initialization. They are then lazily completed when ``requiresCompleteType``
 is called later in Sema.
 
-Resource types are templated class declarations. The template parameter
-specifies the expected return type of resource loads, and the expected parameter
-type for stores.
-
-In Clang's AST and code generation, resource types are classes that store a
-pointer of the template parameter type. The pointer is populated from a call to
-``__builtin_hlsl_create_handle``, and treated as a pointer to an array of typed
-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.
+Resource types are classes that have the "intangible" resource handle type,
+`__hlsl_resource_t`, as a member. These are generally templated class
+declarations that specify the type of data that can be loaded from or stored
+into the resource. The handle is annotated with hlsl-specific attributes
+describing properties of the resource. Member functions of a resource type are
+generally fairly simple wrappers around builtins that operate on the handle
+member.
+
+During code generation resource types are lowered to 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.