[foreign-jextract] RFR: 8252931: Extend multiple constant classes suppor to ClassConstantHelper

[Jorn Vernee]

Hi,

This PR extracts the multiple source file functionality from SourceConstantHelper into a decorator class, and then uses
it for ClassConstantHelper as well.

(I've also done a minor cleanup where I simplified the way booleans are turned into constants, since we can now just
call describeConstable() on a Boolean)

Thanks,
Jorn

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Commit messages:
 - Extract multiple constant classes functionality from SourceConstantHelper and extend it's use to ClassConstantHelper

Changes: https://git.openjdk.java.net/panama-foreign/pull/314/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=314&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252931
  Stats: 249 lines in 6 files changed: 161 ins; 52 del; 36 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/314.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/314/head:pull/314

PR: https://git.openjdk.java.net/panama-foreign/pull/314