[Concurrency] Two isolation inference changes for default isolation mode.

[hborla]

1. Non-actor initializers should participate in custom default isolation inference. Otherwise, they will not be able to touch the type's stored properties to initialize them if those properties have the default isolation inferred.
2. All declarations inside actors and distributed actors do not participate in default isolation inference; they remain unspecified, so the existing isolation inference rules apply.

[hborla]

@swift-ci please smoke test