jextract does not handle VLAs/Incomplete Arrays/Flexible arrays

Historically, jextract has skipped generating layouts for structs and unions that contain incomplete arrays, since clang considered layouts that contain these to be 'incomplete', so trying to retrieve information about other fields would throw an error.

However, even with an incomplete array present, it is still possible to retrieve field offset and size (e.g. using the offsetof macro). So, clang was patched to not throw these errors anymore [1], and we can start taking advantage of this.

[1] : https://reviews.llvm.org/D61239