[OpenCL] Move remaining defines to opencl-c-base.h

Move any remaining preprocessor defines from `opencl-c.h` to
`opencl-c-base.h`, such that they are shared with
`-fdeclare-opencl-builtins` too.

In particular, move:
 - the `as_type` and `as_typen` definitions, and
 - the `kernel_exec` and `__kernel_exec` definitions.

Also clang-format the changes.

Differential Revision: https://reviews.llvm.org/D96948