[Docs] [HLSL] Add IR reference for HLSL

HLSL uses a variety of named IR metadata and attributes to convey
additional information from the frontend to the backend. This document
tries to capture and document the named annotations to provide a
reference for future contributors.

Reviewed By: python3kgae

Differential Revision: https://reviews.llvm.org/D134304

clang/docs/HLSL/HLSLDocs.rst

@@ -11,5 +11,6 @@ HLSL Design and Implementation
 .. toctree::
    :maxdepth: 1
 
+   HLSLIRReference
    ResourceTypes
    EntryFunctions

clang/docs/HLSL/HLSLIRReference.rst

@@ -0,0 +1,31 @@
+=================
+HLSL IR Reference
+=================
+
+.. contents::
+   :local:
+
+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
+===================
+
+``hlsl.shader``
+---------------
+
+The ``hlsl.shader`` function attribute is a string attribute applied to entry
+functions. The value is the string representation of the shader stage (i.e.
+``compute``, ``pixel``, etc).