[OpenCL] Introduce new language options for OpenCL keywords.

OpenCL keywords 'pipe' and 'generic' are unconditionally
supported for OpenCL C 2.0 or in OpenCL C++ mode. In OpenCL C 3.0
these keywords are available if corresponding optional core
feature is supported.

Reviewed By: Anastasia, svenvh

Differential Revision: https://reviews.llvm.org/D95778