JDK 16 RFR of JDK-8235496 : "Start of release updates for JDK 16" and related work

[Joe Darcy]

Hello,

The start of JDK 16 is right around the corner with JDK 15 forking off 
from jdk/jdk at ramp down 1 [1].

Please review the usual javac-related portions of

     http://cr.openjdk.java.net/~darcy/8235496.4/

and the associated CSRs:

     JDK-8245592: Add SourceVersion.RELEASE_16
     https://bugs.openjdk.java.net/browse/JDK-8245592

     JDK-8245586: Add source 16 and target 16 to javac
     https://bugs.openjdk.java.net/browse/JDK-8245586

The update mostly follows the now-expected pattern. The webrev above 
contains a fix for

         JDK-8245544: 
com/sun/tools/javac/platform/JDKPlatformProvider.java does not support 
ct.sym with platform versions 16+

from Jan; this change may be pushed into JDK 15 before the fork. If so, 
it will be sync'ed out of the start-of-16 changes, etc.

Related review threads started on build-dev and hotspot dev.

Thanks,

-Joe

[1] http://openjdk.java.net/projects/jdk/15/