join lines: convert end-of-line comments to block comments when possible (IDEABKL-290)

java/java-tests/testData/codeInsight/joinLines/ConvertComment.java

@@ -0,0 +1,7 @@
+class Foo {
+  void f() {
+    if (ff) <caret>{ //comment
+      return null;
+    }
+  }
+}

java/java-tests/testData/codeInsight/joinLines/ConvertComment_after.java

@@ -0,0 +1,6 @@
+class Foo {
+  void f() {
+    if (ff) { /*comment*/ return null;
+    }
+  }
+}