RFR: 8370687: Improve before constructor has been called error message

[Chen Liang]

In the Valhalla EA2 preview, Value class and record constructor body is in prologue by default, as opposed to all existing constructors and all identity constructors in EA2. This causes a confusion because the error message `cant.ref.before.ctor.called` can appear in such constructors without explicit constructor invocations.

After discussion with @vicente-romero-oracle, we decide to commit this change onto mainline: these two errors never happen in constructors without explicit constructor invocation (colloquially "this" or "super" calls) on mainline, so this message update would not create new confusions. This is future-proof, and is more consistent with the related `call.must.only.appear.in.ctor` message, which is "explicit constructor invocation may only appear within a constructor body".

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Commit messages:
 - 8370687: Improve before constructor has been called error message

Changes: https://git.openjdk.org/jdk/pull/28007/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28007&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370687
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28007.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28007/head:pull/28007

PR: https://git.openjdk.org/jdk/pull/28007