[OpenCL] Add missing C++ legacy atomics with generic

https://reviews.llvm.org/D62335 added some C++ for OpenCL specific
builtins to opencl-c.h, but these were not mirrored to the TableGen
builtin functions yet.

The TableGen builtins machinery does not have dedicated version
handling for C++ for OpenCL at the moment: all builtin versioning is
tied to `LangOpts.OpenCLVersion` (i.e., the OpenCL C version).  As a
workaround, to add builtins that are only available in C++ for OpenCL,
we define a function extension guarded by the __cplusplus macro.

Differential Revision: https://reviews.llvm.org/D100935

Fixes PR50041.