Iterate through symbols once in checkOverrideClashes to improve performance

Original JDK issue: https://bugs.openjdk.java.net/browse/JDK-8224161
Fix in JDK: http://hg.openjdk.java.net/jdk/jdk/rev/28f4594b19c2
Related IJI issue: https://youtrack.jetbrains.com/issue/IJI-167
1 file changed