In the included test case, conformance checking of Wrapper : B would
pick up typealias Foo as a witness for the associated type B.Foo.
However, this typealias Foo is defined in a constrained extension where
T : A, and the underlying type references the associated type A.Foo
on T.
The resulting substitution is invalid when the conformance Wrapper : B
is used in a context where T does not conform to A.
Instead, we should ignore this typealias entirely, since it appears
in an unusable constrained extension.
Fixes <rdar://problem/60219705>, <https://bugs.swift.org/browse/SR-12327>,
<https://bugs.swift.org/browse/SR-12663>.
