RFR: JDK-8310861: Improve location reporting for javac serial lint warnings

[Joe Darcy]

For a subset of the expanded serial lint warnings ([JDK-8202056](https://bugs.openjdk.org/browse/JDK-8202056), [JDK-8160675](https://bugs.openjdk.org/browse/JDK-8160675)), javac uses the location of the enclosing type rather than the method or field in question.

This PR addresses this by using the idiom of finding the method/field's tree within the type tree being checked.

All langtools regression tests pass when run locally; the intention is to backport this fix to JDK 21 as well.

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Commit messages:
 - JDK-8310861: Improve location reporting for javac serial lint warnings

Changes: https://git.openjdk.org/jdk/pull/14644/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14644&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310861
  Stats: 46 lines in 9 files changed: 21 ins; 0 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/14644.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14644/head:pull/14644

PR: https://git.openjdk.org/jdk/pull/14644