RFR: JDK-8226585: Improve javac messages for using a preview API

[Jan Lahoda]

Hi,

This is a continuation of Joe's patch from here:
https://mail.openjdk.java.net/pipermail/compiler-dev/2019-June/013498.html

APIs associated with preview features are split into two groups: 
essential and non-essential. These are marked with an JDK-internal 
annotation, PreviewFeature, and a tag in the javadoc, @preview. The 
javac follows the PreviewFeature annotation, and produces either 
warnings or errors for the usages of such APIs. For the @preview tag, 
there is a taglet in the JDK build that adds the content of the tag into 
the documentation. The first part of the @preview's text goes into the 
summary, the second part goes into the detailed description.

For build, a tricky problem is that the jdk.compiler module uses the 
PreviewFeature annotation as well, but that is not in the bootstrap JDK. 
So, for the intermediate langtools build, the PreviewFeature annotation 
is copied from java.base.

Proposed webrev:
http://cr.openjdk.java.net/~jlahoda/8226585/webrev.00/

Javadoc with the change:
http://cr.openjdk.java.net/~jlahoda/8226585/docs.00/api/index.html

See for example:
http://cr.openjdk.java.net/~jlahoda/8226585/docs.00/api/java.base/java/lang/String.html
http://cr.openjdk.java.net/~jlahoda/8226585/docs.00/api/jdk.compiler/com/sun/source/tree/CaseTree.html

JBS:
https://bugs.openjdk.java.net/browse/JDK-8226585

CSR:
https://bugs.openjdk.java.net/browse/JDK-8231411

Feedback is welcome!

Thanks,
     Jan