RFR: 8278068: Fix next-line modifier (snippet markup)

[Pavel Rappo]

This PR fixes three interrelated issues:

  * https://bugs.openjdk.java.net/browse/JDK-8277027
  * https://bugs.openjdk.java.net/browse/JDK-8278068
  * An accidentally found bug in markup diagnostics

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Commit messages:
 - Fix diagnostic position
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6739/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6739&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278068
  Stats: 218 lines in 7 files changed: 182 ins; 17 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6739.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6739/head:pull/6739

PR: https://git.openjdk.java.net/jdk/pull/6739