SI-8024 Fix inaccurate message on overloaded ambiguous ident

`Symbol#owner` of an overloaded symbol doesn't necessarily correspond
to the owner of any of the alternatives, and as such it shouldn't
be used in error message.

neg/t8024.scala actually represents a progression since 2.10.3;
the ambiguity was not reported. I bisected the change to
https://github.com/scala/scala/pull/1554.