[foreign-jextract] RFR: 8251893: jextract does not generate source for ConstantHelper even when --source is used

[Athijegannathan Sundararajan]

ConstantHelper.java is generated when --source is used

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Commit messages:
 - fixed String constant generation
 - 8251893: jextract does not generate source for ConstantHelper even when --source is used

Changes: https://git.openjdk.java.net/panama-foreign/pull/283/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/283/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251893
  Stats: 1572 lines in 8 files changed: 1058 ins; 487 del; 27 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/283.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/283/head:pull/283

PR: https://git.openjdk.java.net/panama-foreign/pull/283