[foreign-jextract] RFR: 8240300: jextract produces non compilable code with repeated declarations

[Athijegannathan Sundararajan]

Filtering repeated global variables and functions in code generator (added necessary hashCode, equals for decls, types)

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Commits:
 - f1720cf0: 8240300: jextract produces non compilable code with repeated declarations

Changes: https://git.openjdk.java.net/panama-foreign/pull/37/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/37/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240300
  Stats: 289 lines in 5 files changed: 267 ins; 0 del; 22 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/37.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/37/head:pull/37

PR: https://git.openjdk.java.net/panama-foreign/pull/37