RFR: 8372948: Store end positions directly in JCTree

[Liam Miller-Cushon]

This change adds a field to `JCTree` to store end positions, instead of using a separate `EndPosTable` map. See also [this compile-dev@ thread](https://mail.openjdk.org/pipermail/compiler-dev/2025-November/032254.html).

I performed the refactoring in stages, preserving existing semantics at each step.

There are two known places where this changes existing behaviour that are reflected in changes to tests:

* `test/langtools/tools/javac/api/TestJavacTask_Lock.java` - this test asserts that calling `JavacTask#parse` first and then calling `#call` or `#parse` second will fail. The assertion that the test is currently expecting is thrown when the `EndPosTable` gets set a second time, and this change means that no longer results in an exception. If desired `JavacTask#parse` could be updated to explicitly check if it is called twice and fail, instead of indirectly relying on the `EndPosTable` for that.

* `test/langtools/tools/javac/diags/DiagnosticGetEndPosition.java` - there's a comment that 'ideally would be "0", but the positions are not fully set yet', and with the new approach the end position is available to the test, so it resolves the comment

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Commit messages:
 - 8372948: Store end positions directly in JCTree

Changes: https://git.openjdk.org/jdk/pull/28610/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28610&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372948
  Stats: 563 lines in 33 files changed: 7 ins; 437 del; 119 mod
  Patch: https://git.openjdk.org/jdk/pull/28610.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28610/head:pull/28610

PR: https://git.openjdk.org/jdk/pull/28610