Rollup merge of #66654 - ecstatic-morse:check-consts-ref, r=eddyb,matthewjasper

Handle const-checks for `&mut` outside of `HasMutInterior`

Addresses [this comment](https://github.com/rust-lang/rust/pull/64470#discussion_r328200508).

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 `static`s, 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.

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