Fix #9109: More precise override check

[smarter]

Use Denotation#matches instead of Types#matches, the former takes
signatures into account, in the testcase this is needed to ensure that
RefChecks does not emit an error about
def foo[T <: Serializable](x: T): Unit being an invalid override of
def foo[T <: Cloneable](x: T): Unit.

Note that our treatment of polymorphic methods differs from Scala 2
which seems to consider that two polymorphic methods with the same
number of type and term parameters always match and cannot be
overloads (see #8929), but it's closer to what Java does and allows us
to override some Java methods which scalac can't.

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