[OpenCL] Add support of __opencl_c_pipes feature macro.

'pipe' keyword is introduced in OpenCL C 2.0: so do checks for OpenCL C version while
parsing and then later on check for language options to construct actual pipe. This feature
requires support of __opencl_c_generic_address_space, so diagnostics for that is provided as well.

Reviewed By: Anastasia

Differential Revision: https://reviews.llvm.org/D106748