[OpenCL] Add doc to describe OpenCL support

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

Patch by Anastasia Stulova.

Differential Revision: https://reviews.llvm.org/D69072

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@375111 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed