[OpenCL] Guard 64-bit atomic types

Until now, overloads with a 64-bit atomic type argument were always
made available with `-fdeclare-opencl-builtins`.  Ensure these
overloads are only available when both the `cl_khr_int64_base_atomics`
and `cl_khr_int64_extended_atomics` extensions have been enabled, as
required by the OpenCL specification.

Differential Revision: https://reviews.llvm.org/D119858