[foreign-jextract] RFR: 8260583: jextract generates redundant constant files

[Maurizio Cimadamore]

When testing performances of jextract generated code with big headers, I noticed that jextract was emitting empty constant files. After some debugging, I realized that the issues lies in the fact that `ConstantHelper::build` is called both by `ToplevelBuilder::build` and by `HeaderFileBuilder::build` - so, for every new header file split that is generated, we trigger a new constant file, even though that is not needed.

The solution is to remove the call from `HeaderFileBuilder` and just keep the one inside `TopLevelBuilder`.

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Commit messages:
 - Fix issue in constant file generation

Changes: https://git.openjdk.java.net/panama-foreign/pull/440/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=440&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260583
  Stats: 78 lines in 3 files changed: 75 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/440.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/440/head:pull/440

PR: https://git.openjdk.java.net/panama-foreign/pull/440