Rollup merge of #81556 - nikomatsakis:forbidden-lint-groups-lint, r=pnkfelix

introduce future-compatibility warning for forbidden lint groups

We used to ignore `forbid(group)` scenarios completely. This changed in #78864, but that led to a number of regressions (#80988, #81218).

This PR introduces a future compatibility warning for the case where a group is forbidden but then an individual lint within that group is allowed. We now issue a FCW when we see the "allow", but permit it to take effect.

r? ``@Mark-Simulacrum``

diff --cc compiler/rustc_lint/src/context.rs
Simple merge

diff --cc compiler/rustc_lint_defs/src/builtin.rs
Simple merge