[foreign-jextract] RFR: 8252700: jextract ignores typedef to undefined structs

[Athijegannathan Sundararajan]

A class with annotated allocatePointer methods is generated

-------------

Commit messages:
 - 8252700: jextract ignores typedef to undefined structs

Changes: https://git.openjdk.java.net/panama-foreign/pull/307/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=307&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252700
  Stats: 134 lines in 6 files changed: 107 ins; 3 del; 24 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/307.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/307/head:pull/307

PR: https://git.openjdk.java.net/panama-foreign/pull/307