support covariant parameter overrides in AST-based summaries

[jmesserly]

I'm about to send support for @covariant on parameters, which fixes #25578. In the process @stereotype441 helped me get things passing with summaries, however there is a bug, quoting Paul:

The bug they will see while they wait for my fix will be that if they inherit from a class in another package that in turn inherits from a class that declares a parameter as @covariant, the covariant-ness won't get properly passed through (because the intermediate class didn't record inheritsCovariant properly).

We think we can live with that for about a week or so, but filing this bug to track it.

For context, the initial CL for @covariant is https://codereview.chromium.org/2336503003.