[Docs] Add HLSL ResourceType documentation

[beanz]

Summary

Along with the new documentation this also re-organizes the HLSL docs
to a subdirectory. The hope is to continue to expand this documentation
as the HLSL implementation advances.

[aaron.ballman]

LGTM!

[MaskRay]

I'll assume that you have done ninja docs-clang-html:)

clang/docs/HLSL/HLSLDocs.rst

+.. title:: Clang HLSL Documentation
+
+.. toctree::
+   :maxdepth: 1
+
+   HLSLSupport
+
+HLSL Design and Implementation
+==============================
+
+.. toctree::
+   :maxdepth: 1
+
+   ResourceTypes

clang/docs/HLSL/ResourceTypes.rst

+============
+HLSL Support
+============
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+HLSL Resources are runtime-bound data that is provided as input, output or both
+to shader programs written in HLSL. Resource Types in HLSL provide key user
+abstractions for reading and writing resource data.
+
+Implementation Details
+======================
+
+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.

clang/docs/index.rst

.. toctree::
    ShadowCallStack
    SourceBasedCodeCoverage
    Modules
    MSVCCompatibility
    MisExpect
    OpenCLSupport
    OpenMPSupport
    SYCLSupport
-   HLSLSupport
+   HLSL/HLSLDocs
    ThinLTO
    APINotes
    DebuggingCoroutines
    CommandGuide/index
    FAQ
 
 Using Clang as a Library
 ========================