Add SourceVersion.RELEASE_11

Summary

Add a new enum constant RELEASE_11 to javax.lang.model.SourceVersion for the JDK 11 release.

Problem

The SourceVersion enum needs an enum constant for each release being modeled.

Solution

Add the enum constant RELEASE_11.

Specification

--- old/src/java.compiler/share/classes/javax/lang/model/SourceVersion.java 2017-12-11 22:32:47.930062197 -0800
+++ new/src/java.compiler/share/classes/javax/lang/model/SourceVersion.java 2017-12-11 22:32:47.586062182 -0800
@@ -159,7 +159,15 @@
      *
      * @since 10
      */
-     RELEASE_10;
+     RELEASE_10,
+
+    /**
+     * The version recognized by the Java Platform, Standard Edition
+     * 11.
+     *
+     * @since 11
+     */
+     RELEASE_11;

Also, the output of SourceVersion.latest() is changed to RELEASE_11.