[Docs] [HLSL] Add IR reference for HLSL

[beanz]

Summary

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.

[python3kgae]

clang/docs/HLSL/HLSLIRReference.rst

13

Once we document a metadata attribute, is it OK to update the format later?

[beanz]

clang/docs/HLSL/HLSLIRReference.rst

13

Absolutely, none of this is a binding ABI, this document is just intended to be a reference.

[python3kgae]

clang/docs/HLSL/HLSLIRReference.rst

21

Maybe only external global variables?

[beanz]

clang/docs/HLSL/HLSLIRReference.rst

21

Do they have to be external? static RWBuffer<int> Buf is valid right?

[python3kgae]

clang/docs/HLSL/HLSLIRReference.rst

21

static ones don't need to be in the table.

[beanz]

Updating documentation for hlsl.uavs, thanks @python3kgae!

clang/docs/HLSL/HLSLDocs.rst

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

clang/docs/HLSL/HLSLIRReference.rst

+=================
+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).