[foreign-jextract] RFR: 8244512: jextract throws NPE for a nested struct declaration

[Athijegannathan Sundararajan]

saving/restoring 'old' struct builder in visitScoped.

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Commit messages:
 - 8244512: jextract throws NPE for a nested struct declaration
 - 8244425: primitive typedef names differing only in case mapped to nested classes with names differing only in case

Changes: https://git.openjdk.java.net/panama-foreign/pull/146/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/146/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244512
  Stats: 202 lines in 6 files changed: 184 ins; 10 del; 8 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/146.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/146/head:pull/146

PR: https://git.openjdk.java.net/panama-foreign/pull/146