RFR: JDK-8274729: Define Position.NOPOS == Diagnostic.NOPOS

[Jonathan Gibbons]

Please review a trivial change to formally connect `Position.NOPOS` and `Diagnostic.NOPOS`.

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Commit messages:
 - JDK-8274729: Define Position.NOPOS == Diagnostic.NOPOS

Changes: https://git.openjdk.java.net/jdk/pull/5813/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5813&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274729
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5813.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5813/head:pull/5813

PR: https://git.openjdk.java.net/jdk/pull/5813