[foreign-jextract] RFR: 8245767: jextract crashes with typedef on a opaque struct or union

[Athijegannathan Sundararajan]

Class for struct/union typedef is generated only if definition was seen earlier.
Or else it is saved and revisited after all declarations are seen.

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Commit messages:
 - 8245767: jextract crashes with typedef on a opaque struct or union

Changes: https://git.openjdk.java.net/panama-foreign/pull/181/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/181/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8245767
  Stats: 128 lines in 3 files changed: 127 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/181.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/181/head:pull/181

PR: https://git.openjdk.java.net/panama-foreign/pull/181