Expose OpenCL address space qualifiers as type attributes.

OpenCL C standard adds new language keywords to C language to qualify types with address space information (see section 6.1.9 of OpenCL C v2.0 specification).

While working on SYCL implementation (C++-based) we tried to leverage compiler infrastructure developed for OpenCL C, including new keywords. Unfortunately new keywords break compilation of valid SYCL code and can't be used "as-is", so we expose them as "clang specific" type attributes (see https://reviews.llvm.org/D71005). Aaron Ballman proposed to have OpenCL specific type attributes, so we can re-use them for both: OpenCL and SYCL compilers.

WG14 has accepted C++ attribute syntax to C2x and address space qualifiers can be added to the language as OpenCL specific attributes: `[[opencl::private]]`, `[[opencl::local]]`, `[[opencl::constant]]` and `[[opencl::global]]`.

I'd like to know OpenCL WG opinion on whether this makes sense and if so, can we add this spelling to the specification?

Start with section 6.1.9 of the OpenCL C v2.0 specification and the linked LLVM review D71005. Compare the proposed [[opencl::...]] spellings with the existing qualifiers, then determine whether the OpenCL WG accepts adding this syntax to the specification; the issue does not name files or tests.

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