[OpenCL] Change extension handling for -fdeclare-opencl-builtins

Until now, the `-fdeclare-opencl-builtins` option behaved differently
compared to inclusion of `opencl-c.h`: builtins that are part of an
extension were only available if the extension was enabled using the
corresponding pragma.

Builtins that belong to an extension are guarded using a preprocessor
macro (that is named after the extension) in `opencl-c.h`.  Align the
behaviour of `-fdeclare-opencl-builtins` with this.

Co-authored-by: Anastasia Stulova

Differential Revision: https://reviews.llvm.org/D95616