Currently there is no way to tell whether an IR module was generated
using `-cl-std=cl3.0` or `-cl-std=clc++2021`, i.e., whether the origin
was a OpenCL C or C++ for OpenCL source.
Add new `opencl.cxx.version` named metadata when compiling C++. Keep the
`opencl.ocl.version` metadata to convey the compatible OpenCL C version.
Fixes https://github.com/llvm/llvm-project/issues/91912
