There already exists cl_khr_fp64 extension. So OpenCL C 3.0
and higher should use the feature, earlier versions still
use the extension. OpenCL C 3.0 API spec states that extension
will be not described in the option string if corresponding
optional functionality is not supported (see 4.2. Querying Devices).
Due to that fact the usage of features for OpenCL C 3.0 must
be as follows:
```
$ clang -Xclang -cl-ext=+cl_khr_fp64,+__opencl_c_fp64 ...
$ clang -Xclang -cl-ext=-cl_khr_fp64,-__opencl_c_fp64 ...
```
e.g. the feature and the equivalent extension (if exists)
must be set to the same values
Reviewed By: Anastasia
Differential Revision: https://reviews.llvm.org/D96524
Anton Zabaznov