[OpenCL] Change default standard version to CL1.2

Set default version for OpenCL C to 1.2. This means that the
absence of any standard flag will be equivalent to passing
'-cl-std=CL1.2'.

Note that this patch also fixes incorrect version check for
the pointer to pointer kernel arguments diagnostic and
atomic test.

Differential Revision: https://reviews.llvm.org/D106504