JDK 9 RFR of JDK-8161500: Use getTypeName and StringJoiner in core reflection toString methods

Hello, Please review this straightforward cleanup of some of the core reflection implementation classes: JDK-8161500: Use getTypeName and StringJoiner in core reflection toString methods http://cr.openjdk.java.net/~darcy/8161500.1/ All java/lang/Class and java/lang/reflect regression tests pass with this patch. (If the fix for JDK-8054213: "Class name repeated in output of Type.toString()" is pushed before this fix, a simple merge will be needed.) Thanks, -Joe