RFR: 8271711: Remove WorkArounds.isSynthetic

[Joe Darcy]

Switch out logic in WorkArounds for a different expression implemented using javax.lang.model.Elements logic.

Langtools regression test suite passes with the changes.

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Commit messages:
 - 8271711: Remove WorkArounds.isSynthetic

Changes: https://git.openjdk.java.net/jdk/pull/4967/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4967&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271711
  Stats: 5 lines in 2 files changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4967.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4967/head:pull/4967

PR: https://git.openjdk.java.net/jdk/pull/4967