RFR: 7903644: typedef of anonymous struct generates a redundant class

[Maurizio Cimadamore]

This PR tweaks jextract so that typedefs of anonymous structs only generate one class (that of the struct).

This required some changes in both `NameMangler` (to make sure that the special naming rule was applied), and then in `OutputFactory`, to make sure that in such case the typedef class is not emitted.

I've tweaked a number of tests which now reflect the "less verbose" jextract output - as such I don't think we need to add further tests.

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

Commit messages:
 - Add comment
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/195/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=195&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903644
  Stats: 35 lines in 5 files changed: 27 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jextract/pull/195.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/195/head:pull/195

PR: https://git.openjdk.org/jextract/pull/195