[ORC] Add a unit test to verify that bound weak symbols can't be overridden.

[ORC] Add a unit test to verify that bound weak symbols can't be overridden.

Weak symbols can be overridden while they're in the NeverSearched state, but
should not be able to be overridden once they've been bound by some lookup.
Historically we guaranteed this by stripping the weak flag once a symbol as
bound, causing it to appear as if it were strong. In ffe2dda29f3 we changed
that behavior to retain weak flags on symbols (to facilitate tracking for
dynamic re-binding during dlopen). This test checks that we still fail as
required after ffe2dda29f3.