[AST] Enable expression of OpenCL language address spaces an attribute

Summary:
Enable a way to set OpenCL language address space using attributes
in addition to existing keywords.

Signed-off-by: Victor Lomuller <victor@codeplay.com>

Reviewers: aaron.ballman, Anastasia

Subscribers: yaxunl, ebevhan, cfe-commits, Naghasan

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71005



Signed-off-by: Alexey Bader <alexey.bader@intel.com>