Previously, constraint wasn't added if bound was ErrorType. That could
cause TypeVariable to be inferred to Any?/Nothing instead of ErrorType,
which could influence other parts of analysis (in particular, inferring
Nothing instead of ErrorType can cause bogus UNREACHABLE_CODE diagnostics)
Because of that fix, intersection type can be added to CS as supertype,
which provokes AssertionError. This commit also relaxes this assertion,
as it seems that it's valid that supertype is an intersection type.
Dmitry Savvinov
eadd1355c2