[OpenCL] Added doc to describe OpenCL support

[Anastasia]

Summary

The idea of this page is to document WIP functionality and also describe the plan of future development work.

[mantognini]

Shouldn't this page be referenced from clang/docs/index.rst?

In the long run, how would this page differ from https://releases.llvm.org/9.0.0/tools/clang/docs/LanguageExtensions.html#opencl-features and https://releases.llvm.org/9.0.0/tools/clang/docs/UsersManual.html#opencl-features ? Do you think the content of those pages should be (partially) moved to the new one?

[Anastasia]

In D69072#1712473, @mantognini wrote:

Shouldn't this page be referenced from clang/docs/index.rst?

Oops, yes! Sure. I will update. Thanks!

In the long run, how would this page differ from https://releases.llvm.org/9.0.0/tools/clang/docs/LanguageExtensions.html#opencl-features

This documents the language extensions.

and https://releases.llvm.org/9.0.0/tools/clang/docs/UsersManual.html#opencl-features ?

Explains how to use OpenCL

Do you think the content of those pages should be (partially) moved to the new one?

Possibly but the page I am adding is intended to document the state of implementations i.e. features that are incomplete or in intermediate phase and potentially even what we are planning to support in the near future. So it's intended more for the clang users to understand what they can or can't compile yet.

[Anastasia]

Added OpenCLSupport page into index.

[mantognini]

Alright, thanks for the explanation. LGTM then.

clang/docs/OpenCLSupport.rst

+.. raw:: html
+
+  <style type="text/css">
+    .none { background-color: #FFCCCC }
+    .partial { background-color: #FFFF99 }
+    .good { background-color: #CCFF99 }
+  </style>
+
+.. role:: none
+.. role:: partial
+.. role:: good
+
+.. contents::
+   :local:
+
+==================
+OpenCL Support
+==================
+
+Clang fully supports all OpenCL C versions from 1.1 to 2.0.
+
+Please refer to `Bugzilla
+<https://bugs.llvm.org/buglist.cgi?component=OpenCL&list_id=172679&product=clang&resolution=--->`_
+for the most up to date bugs report.
+
+
+C++ for OpenCL Implementation Status
+====================================
+
+Bugzilla bugs for this functionality are typically prefixed
+with '[C++]'.
+
+Differences to OpenCL C
+-----------------------
+
+TODO!
+
+Missing features or with limited support
+----------------------------------------
+
+- Use of ObjC blocks is disabled.
+
+- Global destructor invocation is not generated correctly.
+
+- Initialization of objects in `__constant` addr spaces is not guaranteed to work.
+
+- `addrspace_cast` operator is not supported.