RFR: 8242564: javadoc crashes:: class cast exeception com.sun.tools.javac.code.Symtab$6

[Vladimir Petko]

'--ignore-source-errors' allows generating Javadoc for the packages that contain compilation errors. 

jdk.javadoc.internal.doclets.toolkit.util.ClassTree generates a type hierarchy for javadoc that may include error types such as 

class Foo extends Bar {
}
``` 
where Bar is undefined. 

The user still wants to generate documentation for Foo and have Bar as a text label. 

For the unknown class Bar it is impossible to detect the enclosing class/file and javadoc crashes with exception. 

This PR returns Kind.OTHER for the error types, avoiding the javadoc crash.

-------------

Commit messages:
 - Move test comments
 - Use toolbox to write the test file
 - add text block indentation
 - Return Kind.OTHER for undefined symbols
 - Add ClassTree test with undefined symbols

Changes: https://git.openjdk.org/jdk/pull/17435/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17435&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8242564
  Stats: 58 lines in 2 files changed: 53 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/17435.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17435/head:pull/17435

PR: https://git.openjdk.org/jdk/pull/17435