feat: show what lint was overruled
We can't `#[allow]` a whole lint group if any of its members is forbidden, but the offending member is not currently shown if it was forbidden from the command line.
Before/after:
```diff
$ rustc -F dead_code - <<< '#![allow(unused)]'
error[E0453]: allow(unused) incompatible with previous forbid
--> <anon>:1:10
|
1 | #![allow(unused)]
| ^^^^^^ overruled by previous forbid
|
- = note: `forbid` lint level was set on command line
+ = note: `forbid` lint level was set on command line (`-F dead_code`)
error: aborting due to 1 previous error
```
@rustbot label +A-diagnostics +A-lints +D-terse
139651428d