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

[Joseph D. Darcy]

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