Auto merge of #6098 - longlb:interior_mut_const, r=ebroto

Downgrade interior_mutable_const lints to warn by default

This change updates the two lints in the file non_copy_const.rs to be warn by default rather than deny by default. It also updates the known problems for declare_interior_mutable_const to mention some issues that are affected by the lints.

This is a repeat pull request since I botched the first one (#6012). Apart from my messing up the commits of that one, I also had a problem where the stderr of the tests didn't change despite me changing both lints to warn by default. Is this normal behaviour for some lints or do I need to adjust the tests to accommodate the change?

fixes #5863
changelog: none