When a witness is unsafe and its corresponding requirement is not
unsafe, the conformance itself needs to be determined to be unsafe.
When doing this check, account for the fact that the requirement might
be implicitly unsafe, i.e., it uses unsafe types. In such cases, the
requirement is effectively unsafe, so the unsafe witness to it does not
make the conformance unsafe. Therefore, suppress the diagnostic. This
accounts for cases where the protocol comes from a module that didn't
enable strict memory safety checking, or didn't suppress all warnings
related to it, as well as cases where substitution of type witnesses
introduces the unsafe type.
The same thing occurs with overriding a method: an unsafe override of
a method that involves unsafe types (but was not marked @unsafe for
whatever reason) does not make the subclassing itself unsafe, so don't
diagnose it as such.
As we do when referencing other kinds of declarations, if a
typealias isn't `@unsafe`, but it involves unsafe types,
diagnose the non-safety at the point of reference.
Fixes https://github.com/swiftlang/swift/issues/78220
When referencing a declaration, check whether any of the types in that
reference are unsafe. This can diagnose cases where the original
declaration either wasn't actually unsafe, or is being provided with
unsafe types via generics.
