Fix #59209.

Fix #59209.
* mcs/expression.cs (Invocation.BetterFunction): Remove support for
comparing virtual functions and their overrides.
(Invocation.IsOverride): New.
(Invocation.OverloadResolve): Don't consider 'override' functions
during candidate selection.  Store them in a lookaside list.
If the selected method is a 'virtual' function, use the list to
find any overrides that are closer to the LHS type.

* tests/test-346.cs: New test from #59209.

svn path=/trunk/mcs/; revision=40625