Auto merge of #16816 - MortimerGoro:unsafe_layout_data_cast, r=SimonSapin

Fix unsafe casting between PersistentLayoutData and PartialPersistentLayoutData

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PersistentLayoutData and PartialPersistentLayoutData castings in Layout component assume that they have the same base raw address. This is unsafe because field orders is not guaranteed by default, and it's causing some SEGV crashes on Android indeed (https://github.com/servo/servo/issues/16647)

Add a C representation to guarantee that the order of the fields and the safe casting.
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- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #16647 (github issue number if applicable).

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