RFR: JDK-8296322 javac: use methods to manage parser mode flags

[Jim Laskey]

Manage parser mode flags using methods for easy of reading and maintenance.

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Commit messages:
 - JDK-8296322 javac: use methods to manage parser mode flags

Changes: https://git.openjdk.org/jdk/pull/10981/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10981&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296322
  Stats: 82 lines in 1 file changed: 17 ins; 0 del; 65 mod
  Patch: https://git.openjdk.org/jdk/pull/10981.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10981/head:pull/10981

PR: https://git.openjdk.org/jdk/pull/10981