Fix inconsistencies with isolated parameters.

[kavon]

subsumes #60483

There are a whole bunch of ways the rules about isolated parameters are either not being enforced, or the wrong isolation is being used for such functions. This PR tries to sharpen the definition and rules so they make sense.

TODOs:

• maybe move the check to TypeChecker::checkParameterList
• fix isolation for subscript (seems to default to nonisolated) deferred.
• make sure isolation for initializers is working / covered (doesn't check isolation at all?) deferred.
• fix isolation for function values like this:

@MainActor
func update<R>(_ callback: @Sendable @escaping @MainActor (isolated Foo) -> R) -> R {
  callback(Foo()) // error: call to actor-isolated parameter 'callback' in a synchronous main actor-isolated context
}

• enforce the rules for closures

resolves rdar://100039019

[kavon]

@swift-ci please test

[ktoso]

Looking good so far! I am confused about: 46fd8e2 (#61862) this is not the semantics I'd expect there.

[ktoso]

Resolves #60474

[kavon]

@swift-ci please test

[kavon]

@swift-ci please test and merge