[jdk21u] RFR: 8315452: Erroneous AST missing modifiers for partial input

[Jan Lahoda]

This is a backport of:
https://github.com/openjdk/jdk/commit/84425a62904f84601affc9710eefece88665374a

The change in `JavacParser` backported cleanly, the change in the test required a very simple merge, due to the branches diverging.

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Commit messages:
 - 8315452: Erroneous AST missing modifiers for partial input

Changes: https://git.openjdk.org/jdk21u/pull/131/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=131&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315452
  Stats: 65 lines in 2 files changed: 61 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk21u/pull/131.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/131/head:pull/131

PR: https://git.openjdk.org/jdk21u/pull/131