The diagnostic for non-Sendable globa/static `let` properties was checking
for a Sendable conformance without considering `@preconcurrency`. Emit
this diagnostic via a `@preconcurrency`-sensitive path.
Fixes rdar://121889248.
When the conformance checker and override checker detect a difference in a function type’s @Sendable attribute that varies in an illegal way, they now check if the protocol/base class was imported with an @preconcurrency import, and either limit the diagnostic or suggest adding @preconcurrency to the import as appropriate.
Completes rdar://91109455.
If a type is explicitly non-Sendable, we don’t want to recommend softening errors with `@preconcurrency import` (even thought that would still work), because these probably represent *real* errors that you ought to fix. And at the extreme end of this, if a module is built with `-warn-concurrency` or `-swift-version 6`, *all* of its types should be treated as though they have explicit sendability, since we assume the entire module has been audited.
(Eventually, we plan to do this at module build time by actually serializing out unavailable conformances rather than by looking at the settings used to build the module, but we aren’t there quite yet.)
Implement this and test it by checking that the preconcurrency sendable tests never recommend adding `@preconcurrency` to an import of `StrictModule`, even when we do the same illegal things with it that cause that remark to be printed for `NonStrictModule`.
Extend the diagnostics for `Sendable` conformances to always diagnose
missing `Sendable` conformances for nominal types that are within the
same module. The intuition here is that if the type is in the same
module, it can be updated and evaluated at the same time as code
requiring the `Sendable` conformance is introduced.
Another part of rdar://78269348.
