Tolerate symbol sharing between accessor/field.

Recently, TreeCheckers (-Ycheck) was extended to
report on references to symbols that were not in
scope, which is often a sign that some substitution
or ownership changes have been omitted.

But, accessor methods directly use the type of the
underlying field, without cloning symbols defined
in that type, such as quantified types in existentials,
at the new owner.

My attempt to change this broke pos/existentials.scala.

Instead, I'll just look the other way in TreeCheckers.
Merge pull request #2102 from retronym/topic/tree-checker-leniancy

Tolerate symbol sharing between accessor/field.