[OpenCL] Add const, volatile and pointer builtin handling

Const, volatile, and pointer types were previously available, but not
working.  This patch adds handling for OpenCL builtin functions.

Add TableGen definitions for some atomic and asynchronous builtins to
make use of the new functionality.

Patch by Pierre Gondois and Sven van Haastregt.

Differential Revision: https://reviews.llvm.org/D63442

llvm-svn: 369373