[OpenCL] Add OpenCL 3.0 atomics to -fdeclare-opencl-builtins

Add the atomic overloads for the `global` and `local` address spaces,
which are new in OpenCL 3.0.  Ensure the preexisting `generic`
overloads are guarded by the generic address space feature macro.

Ensure a subset of the atomic builtins are guarded by the
`__opencl_c_atomic_order_seq_cst` and `__opencl_c_atomic_scope_device`
feature macros, and enable those macros for SPIR/SPIR-V targets in
`opencl-c-base.h`.

Also guard the `cl_ext_float_atomics` builtins with the atomic order
and scope feature macros.

Differential Revision: https://reviews.llvm.org/D119420