RFR: 7903239: ofAddress factory of function pointer type is wrong for struct returns [v2]

[Marko]

> When generating the lambda inside the `ofAddress` factory, it is missing the `SegmentAllocator` needed when the function returns a struct. This is [implemented for regular functions](https://github.com/openjdk/jextract/blob/0582eaf1b4cdba95f0ee8c2480767433bb647d0d/src/main/java/org/openjdk/jextract/impl/HeaderFileBuilder.java#L122-L127), but not for function pointers.

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

  add proper tests

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/58/files
  - new: https://git.openjdk.org/jextract/pull/58/files/85c002ae..896aeabe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=58&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=58&range=00-01

  Stats: 162 lines in 5 files changed: 95 ins; 65 del; 2 mod
  Patch: https://git.openjdk.org/jextract/pull/58.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/58/head:pull/58

PR: https://git.openjdk.org/jextract/pull/58