[jdk21u-dev] RFR: 8242564: javadoc crashes:: class cast exception com.sun.tools.javac.code.Symtab$6

[Todd V. Jonker]

This is a clean backport of https://github.com/openjdk/jdk/commit/64c3642c57719940855b220025b33758950b3980 originally authored by @vpa1977 and reviewed by @jonathan-gibbons 

Additional testing:
- [x] New regression test fails without the fix, passes with it
- [x] linux-x86_64-server-release: `make test TEST=test/langtools/jdk/javadoc`

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Commit messages:
 - Backport 64c3642c57719940855b220025b33758950b3980

Changes: https://git.openjdk.org/jdk21u-dev/pull/856/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=856&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8242564
  Stats: 61 lines in 2 files changed: 56 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/856.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/856/head:pull/856

PR: https://git.openjdk.org/jdk21u-dev/pull/856