Fix #7630: Use weak conformance in overloading resolution

[odersky]

Following the spec, use weak conformance when determining whether an
overloaded variant is applicable to arguments.

[sjrd]

The changes in the tests of harmonization are indeed necessary because Array.apply is overloaded. However, they do constitute a regression in test coverage. We can reclaim the test coverage by using another invariant collection whose apply is not overloaded. For example we can use ArrayBuffer. I suggest to copy-paste the old tests with Arrays and adapting the copy to use ArrayBuffer instead. The ArrayBuffer tests should exhibit the same harmonization behavior as before.