In SqlWalker::walkJoin(), SqlWalker::walkRangeVariableDeclaration() can be
called which may produce an 'ON' clause if the entity inheritance type is
JOINED. As walkJoin() may then produce another ON clause, this results in
invalid SQL (e.g. '... ON foo = bar ON (baz = quux) ...' when the inheritance
type is JOINED.
This adds a test and a fix for the problem, by checking for an inheritance type
of JOINED in walkJoin() and using AND instead of ON in the appropriate place.
It seems like this part of the code is begging to be refactored. This is my
first foray into Doctrine internals and can't see a way to do this without
stomping all over the rest of the code, but this section seems ripe for cleanup
by somebody who is familiar.