RFR: 7904051: Jextract should generate symbols in the same class consistently

[Nizar Benalla]

Copying the bug description:

> A symbol, depending on the order that clang found it will not necessarily be in the same class every time.
The IDE works against you here, when you navigate to a symbol and it exists in header number 4 for example, it will sometimes automatically import header number 4 so you have to go and change it because you want to get it from the top level class.
>If you don't do that, you build a dependency for your platform into the generated code.
>The symbols not being consistent across platforms can be fixed by sorting the symbols.

Very simple change, done in a way to not break existing code as a simple sort would've changed the names of variables in the generated code.
There was a test case that had to be updated.

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Commit messages:
 - sort symbols and update test case

Changes: https://git.openjdk.org/jextract/pull/286/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=286&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904051
  Stats: 8 lines in 2 files changed: 4 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jextract/pull/286.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/286/head:pull/286

PR: https://git.openjdk.org/jextract/pull/286