RFR: Bring jdk20 branch in sync with panama branch

[Maurizio Cimadamore]

On Wed, 22 Mar 2023 11:50:28 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This PR brings fixes from `panama` branch back to the `jdk20` branch.
> We are doing this as part of getting ready to start shipping binary snapshot of the jdk20 version of jextract.

src/main/java/org/openjdk/jextract/impl/StructLayoutComputer.java line 158:

> 156:                 addField(offset, bitfield(prevBitfieldDecls.toArray(new Declaration.Variable[0])));
> 157:             }
> 158:             if (prevBitfieldSize == 0) {

This `if` is new - there is a change in semantics from Java 20 to Java 21 - as Java 20 disallows creation of zero-length padding, while Java 21 allows it (which seems like a bug). So, to make jextract work against Java 20, this change was required.

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PR Review Comment: https://git.openjdk.org/jextract/pull/114#discussion_r1144676786