[java] since Java 9, underscore identifiers are completely prohibited (IDEA-132278)

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

@@ -0,0 +1,3 @@
+class Undescore {
+  void <error descr="As of Java 9, '_' is a keyword, and may not be used as an identifier">_</error>() { }
+}