dropck: treat parametric types as safe for dropping.

Handles e.g. `impl<T> Drop for Vec<T>` as parametric: If `T` does not
have any drop code that could read from borrowed data of lifetime `'a`,
then we infer that the drop code for `Vec<T>` also cannot read from
borrowed data of lifetime `'a`, and therefore we do not need to inject
the SafeDestructor constraint for it.

Notably, this enables us to continue storing cyclic structure, without
any `unsafe` code, in `Vec`, without allowing (unsound) destructors on
such cyclic data. (Later commits have tests illustrating these two
cases in run-pass and compile-fail, respectively.)

(This is "Condition (B.)" in Drop-Check rule described in RFC 769.)

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