RFR: 7903665: Function pointer code generation issues [v2]

[Maurizio Cimadamore]

> This PR fixes two minor issues with function pointer generation:
> * static is missing from some nested classes (function pointers)
> * function pointers also miss a default constructor

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Update test/testng/org/openjdk/jextract/test/toolprovider/nestedDecls/TestNestedDecls.java
  
  Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/jextract/pull/212/files
  - new: https://git.openjdk.org/jextract/pull/212/files/4721eee2..c3ee2594

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=212&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=212&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/212.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/212/head:pull/212

PR: https://git.openjdk.org/jextract/pull/212