Clear proxy_set cache when creating an annotated column

Clear proxy_set cache when creating an annotated column

Fixed an unlikely issue where the "corresponding column" routine for unions
and other :class:`.CompoundSelect` objects could return the wrong column in
some overlapping column situtations, thus potentially impacting some ORM
operations when set operations are in use, if the underlying
:func:`.select` constructs were used previously in other similar kinds of
routines, due to a cached value not being cleared.

Fixes: #4747
Change-Id: I7fb134cac3604f8fe62e220fb24a0945d0a1c56f