[foreign-jextract] RFR: 8277070: Regularize logic for cursor -> position conversion

[Maurizio Cimadamore]

TreeMaker::toPos has the correct logic to extract a Position from a cursor, which avoids any potential NPEs. Unfortunately jextract code does not use it uniformly, especially `RecordLayoutComputer` and its subclasses. This patch turns the method into a static helper and makes the `CursorPosition` constructor private, so that other clients have to go through the static factory.

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Commit messages:
 - Turn helper function into a factory method in CursorPosition
 - Turn TreeMaker::toPos into a static helper.

Changes: https://git.openjdk.java.net/panama-foreign/pull/613/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=613&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277070
  Stats: 44 lines in 5 files changed: 12 ins; 15 del; 17 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/613.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/613/head:pull/613

PR: https://git.openjdk.java.net/panama-foreign/pull/613