Rollup merge of #147467 - JonathanBrouwer:double_warnings, r=JonathanBrouwer

Fix double warnings on `#[no_mangle]`

Fixes 2 out of 3 cases in rust-lang/rust#147417
The fix on closures removes the old error and marks closures as an error target.
The fix on consts adds `AllowSilent` to to ignore a target, and uses the old error because that one has a nice suggestion.

r? ````@jdonszelmann````

Author: Zalathar