java completion: don't consider underscore lowercase (IDEA-210869)

java/java-tests/testData/codeInsight/completion/className/nameCompletion/java/ClassStartsWithUnderscore-result.java

@@ -0,0 +1,5 @@
+import foo._SomeClass;
+
+class C {
+    _SomeClass<caret>
+}

java/java-tests/testData/codeInsight/completion/className/nameCompletion/java/ClassStartsWithUnderscore-source.java

@@ -0,0 +1,3 @@
+class C {
+    _SomeCl<caret>
+}