Handle const-checks for &mut outside of HasMutInterior

[ecstatic-morse]

Addresses this comment.

Const-checking relied on HasMutInterior to forbid &mut in a const context. This was strange because all we needed to do was look for an Rvalue::Ref with a certain BorrowKind, whereas the Qualif traits are specifically meant to get the qualifs for a value. This PR removes that logic from HasMutInterior and moves it into check_consts::Validator.

As a result, we can now properly handle qualifications for statics, which had to be ignored previously since you can e.g. borrow a static Cell from another static. We also remove the derived_from_illegal_borrow logic, since it is no longer necessary; we give good errors for subsequent reborrows/borrows of illegal borrows.