[jdk17] RFR: JDK-8269150 Unicode \u005C not treated as an escaping backslash

[Jim Laskey]

\u005C Unicode escape sequence not being treated as a backslash for general escape sequences.

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Commit messages:
 - Correct unicode escape handling in UnicodeReader, added test

Changes: https://git.openjdk.java.net/jdk17/pull/126/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=126&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269150
  Stats: 37 lines in 2 files changed: 36 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/126.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/126/head:pull/126

PR: https://git.openjdk.java.net/jdk17/pull/126