We previously would diagnose them as a GNU extension in C mode, but they
are now a feature of C23. The -Wgnu-binary-literal warning group no
longer controls any diagnostics as this is no longer a GNU extension.
The warning group is retained as a noop to help avoid "unknown warning"
diagnostics.
This also adds the companion compatibility warning which existed for C++
but not for C.
Fixes https://github.com/llvm/llvm-project/issues/72017
8e24bc096d
