### What changes were proposed in this pull request?
This PR fixes an issue that `sbt unidoc` fails with JDK11.
With the current master, `sbt unidoc` fails because the generated Java sources cause syntax error.
As of JDK11, the default doclet seems to refuse such syntax error.
Usually, it's enough to specify `--ignore-source-errors` option when `javadoc` runs to suppress the syntax error but unfortunately, we will then get an internal error.
```
[error] javadoc: error - An internal exception has occurred.
[error] (java.lang.NullPointerException)
[error] Please file a bug against the javadoc tool via the Java bug reporting page
[error] (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com)
[error] for duplicates. Include error messages and the following diagnostic in your report. Thank you.
[error] java.lang.NullPointerException
[error] at jdk.compiler/com.sun.tools.javac.code.Types.erasure(Types.java:2340)
[error] at jdk.compiler/com.sun.tools.javac.code.Types$14.visitTypeVar(Types.java:2398)
[error] at jdk.compiler/com.sun.tools.javac.code.Types$14.visitTypeVar(Types.java:2348)
[error] at jdk.compiler/com.sun.tools.javac.code.Type$TypeVar.accept(Type.java:1659)
[error] at jdk.compiler/com.sun.tools.javac.code.Types$DefaultTypeVisitor.visit(Types.java:4857)
[error] at jdk.compiler/com.sun.tools.javac.code.Types.erasure(Types.java:2343)
[error] at jdk.compiler/com.sun.tools.javac.code.Types.erasure(Types.java:2329)
[error] at jdk.compiler/com.sun.tools.javac.model.JavacTypes.erasure(JavacTypes.java:134)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.Utils$5.visitTypeVariable(Utils.java:1069)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.Utils$5.visitTypeVariable(Utils.java:1048)
[error] at jdk.compiler/com.sun.tools.javac.code.Type$TypeVar.accept(Type.java:1695)
[error] at java.compiler11.0.9.1/javax.lang.model.util.AbstractTypeVisitor6.visit(AbstractTypeVisitor6.java:104)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.Utils.asTypeElement(Utils.java:1086)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.LinkInfoImpl.setContext(LinkInfoImpl.java:410)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.LinkInfoImpl.<init>(LinkInfoImpl.java:285)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.LinkFactoryImpl.getTypeParameterLink(LinkFactoryImpl.java:184)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.LinkFactoryImpl.getTypeParameterLinks(LinkFactoryImpl.java:167)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.links.LinkFactory.getLink(LinkFactory.java:196)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.HtmlDocletWriter.getLink(HtmlDocletWriter.java:679)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.HtmlDocletWriter.addPreQualifiedClassLink(HtmlDocletWriter.java:814)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.HtmlDocletWriter.addPreQualifiedStrongClassLink(HtmlDocletWriter.java:839)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.AbstractTreeWriter.addPartialInfo(AbstractTreeWriter.java:185)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.AbstractTreeWriter.addLevelInfo(AbstractTreeWriter.java:92)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.AbstractTreeWriter.addLevelInfo(AbstractTreeWriter.java:94)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.AbstractTreeWriter.addTree(AbstractTreeWriter.java:129)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.AbstractTreeWriter.addTree(AbstractTreeWriter.java:112)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.PackageTreeWriter.generatePackageTreeFile(PackageTreeWriter.java:115)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.PackageTreeWriter.generate(PackageTreeWriter.java:92)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.HtmlDoclet.generatePackageFiles(HtmlDoclet.java:312)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.AbstractDoclet.startGeneration(AbstractDoclet.java:210)
[error] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.AbstractDoclet.run(AbstractDoclet.java:114)
[error] at jdk.javadoc/jdk.javadoc.doclet.StandardDoclet.run(StandardDoclet.java:72)
[error] at jdk.javadoc/jdk.javadoc.internal.tool.Start.parseAndExecute(Start.java:588)
[error] at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:432)
[error] at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:345)
[error] at jdk.javadoc/jdk.javadoc.internal.tool.Main.execute(Main.java:63)
[error] at jdk.javadoc/jdk.javadoc.internal.tool.Main.main(Main.java:52)
```
I found the internal error happens when a generated Java class is from a Scala class which is package private and generic.
I also found that if we don't generate class hierarchy tree in the JavaDoc, we can suppress the internal error for JDK11 and later.
### Why are the changes needed?
Make the build success with sbt and JDK11.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
I confirmed the following command successfully finish with JDK8 and JDK11.
```
$ build/sbt -Phive -Phive-thriftserver -Pyarn -Pkubernetes -Pmesos -Pspark-ganglia-lgpl -Pkinesis-asl -Phadoop-cloud clean unidoc
```
I also confirmed html files are successfully generated under `target/javaunidoc`.
Closes#31023 from sarutak/fix-genjavadoc-java11.
Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
acf0a4fac2