[Checkers] TypesRelations loop

[Matt Papi]

When running the NonNull checker on Daikon (per Mike's instructions in
a previous message) I'm getting a StackOverflowError. Here's the
essential part of the stack trace:

        at checkers.util.TypeRelations.isSubtype(TypeRelations.java:162)
        at checkers.util.TypeRelations.compareTypeArg(TypeRelations.java:56)
        at checkers.util.TypeRelations.compareTypeArguments(TypeRelations.java:117)


Strangely, sometimes it doesn't occur at all, and when it does, it's
happening while checking different files.

(I'm running "javac -typeprocessor checkers.nonnull.NonNullChecker
-Afilenames `findfile '*.java'`" in
~mernst/tmp/nninfer/invariants-java  with a clean checkout + build of
checkers r2242.)


- Matt