[jdk23] RFR: 8334757: AssertionError: Missing type variable in where clause

[Liam Miller-Cushon]

Please review this clean backport of #19840 to JDK 23, which avoids a crash when types with type variables appear as arguments for diagnostics that use `JCDiagnostic.AnnotatedType`.

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Commit messages:
 - Backport babf6df7d97e4beedb25e689634d999412c1e950

Changes: https://git.openjdk.org/jdk/pull/20085/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20085&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334757
  Stats: 24 lines in 3 files changed: 24 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20085.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20085/head:pull/20085

PR: https://git.openjdk.org/jdk/pull/20085