importC: check mixing _Alignas and attribute(aligned)/declspec(align) on computed struct alignment

[ibuclaw]

#21765

Specific to mixing C _Alignas and attribute/declspec aligned .

1. Which alignment wins on regular fields?
2. Ditto bitfields
3. What's its Interaction with packed attribute? Ie: tail padding applied in structs alters position of next field, or static arrays.