Re-entrance of SourceTypeBinding.methods() causes errors to be reported repeatedly

Work on #710 shows that some duplicate error logs are actually *expected* from tests.

One example is `org.eclipse.jdt.core.tests.compiler.regression.AnnotationTest.testBug376429a()` where it turns out that repeated reporting is caused by re-entrant STB.methods() calls.

We have some code complexity in place to cope with this situation, but the fact of duplicate error reports indicates that this may not be fully clean.