RFR: 7903481: Jextract doesn't enforce group layout alignment correctly in some cases

[Maurizio Cimadamore]

This patch overhauls the treatment of pragma packs directives.

The current logic tries to detect fields occurring at misaligned offsets, and relaxes alignment constraints for these fields.
However, in cases like this:


#pragma pack(push, 1)
struct A {
   long long a;
   int b;
}


Each field is correctly aligned. But the struct size (12) is not a multiple of its natural alignment (8). As a result, we run into issues when building a sequence layout out of this struct, because of the eager checks added to the layout API.

This patch fixes support for packed structs "the right way", that is, by asking clang the struct/union alignment, and then making sure that any field is aligned accordingly before the group layout is created.

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

Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/121/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=121&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903481
  Stats: 105 lines in 9 files changed: 79 ins; 6 del; 20 mod
  Patch: https://git.openjdk.org/jextract/pull/121.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/121/head:pull/121

PR: https://git.openjdk.org/jextract/pull/121