[OpenCL] Remove mixed signedness atomic_fetch_ from opencl-c.h

The OpenCL C specification v3.0.6 s6.15.12.7.5 mentions:

    For atomic_fetch and modify functions with key = or, xor, and, min
    and max on atomic type atomic_intptr_t, M is intptr_t, and on
    atomic type atomic_uintptr_t, M is uintptr_t.

Remove the atomic_fetch_* overloads from opencl-c.h that mix intptr_t
and uintptr_t in the same declaration.

Differential Revision: https://reviews.llvm.org/D98418