[OpenCL] Clang diagnostics allow reporting C++ for OpenCL version.

Some Clang diagnostics could only report OpenCL C version. Because
C++ for OpenCL can be used as an alternative to OpenCL C, the text
for diagnostics should reflect that.

Desrciptions modified for these diagnostics:
`err_opencl_unknown_type_specifier`
`warn_option_invalid_ocl_version`
`err_attribute_requires_opencl_version`
`warn_opencl_attr_deprecated_ignored`
`ext_opencl_ext_vector_type_rgba_selector`

Differential Revision: https://reviews.llvm.org/D107648