jdk9: support private methods in interfaces (IDEA-137918)

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/advHighlighting9/PrivateInInterfaces.java

@@ -0,0 +1,3 @@
+interface A {
+    private void m() {}
+}

java/java-tests/testSrc/com/intellij/codeInsight/daemon/LightAdvHighlightingJdk9Test.java

@@ -53,4 +53,5 @@ public class LightAdvHighlightingJdk9Test extends LightDaemonAnalyzerTestCase {
   }
 
   public void testSafeVarargsApplicability() { doTest(true, false); }
+  public void testPrivateInInterfaces() { doTest(false, false); }
 }