RFR: 8334258: Compiler erronousely allows access to instance variable in argument expression of a constructor invocation

[Archie Cobbs]

For the "flexible constructors" JEP 482 feature, the compiler allows assignments to instance fields in early construction contexts. However, when it creates this exception it is failing to check that the feature is enabled. This patch adds that check.

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Commit messages:
 - Disallow early assignments without FLEXIBLE_CONSTRUCTORS feature.

Changes: https://git.openjdk.org/jdk/pull/19712/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19712&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334258
  Stats: 83 lines in 7 files changed: 74 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/19712.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19712/head:pull/19712

PR: https://git.openjdk.org/jdk/pull/19712