This commit changes fixit messages from a question/suggestion to an
imperative message for protocol conformances and switch-case. Addresses
https://github.com/apple/swift/issues/67510.
Isolation checking for calls had two separate implementation places:
one that looked at the declaration being called (for member
declarations) and one that worked on the actual call expression. Unify
on the latter implementation, which is more general and has access to
the specific call arguments. Improve diagnostics here somewher so we
don't regress in that area.
This refactoring shouldn't change the actual semantics, but it makes
upcoming semantic changes easier.
Actor inheritance was removed in the second revision of SE-0306. Remove
the ability to inherit actors.
Note that this doesn't fully eliminate all vestigates of inheritance
from actors. There are simplifications that need to be performed
still, e.g., there's no need to distinguish
designated/convenience/required initializers. That will follow.
