[clang] [OpenCL] Disable __opencl_c_ext_fp64_* features if cl_khr_fp64 is not supported (PR #169252)

[Alexey Bader via cfe-commits]

https://github.com/bader approved this pull request.

Thanks. 
Looks good to me, but I'd like to know what @svenvh thinks.

https://github.com/llvm/llvm-project/pull/169252