The macro changes the body of the initializer-list constructor and adds a
to_json_tuple_impl overload, both with the same mangled names in either
mode, so mixing translation units silently picked one definition. Encode
it in the inline namespace as `_bics`, as JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
does with `_ldvcmp`. The macro is new in the unreleased 3.13.0, so no
existing namespace name changes.
- Move the macro's default into abi_macros.hpp so json_fwd.hpp computes
the same namespace, and keep it defined under JSON_TEST_KEEP_MACROS.
- Check the tag in the ABI config tests and in the unit test.
- List `_bics` (and the missing `_dp`) in the namespace docs and in the
natvis generator; regenerate nlohmann_json.natvis.
- Replace the "define it consistently" warning with an ABI note.
Suggested by @gregmarr in the review of #5544.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>