Define a struct-level `#[safety_constraint(...)]` attribute #7547

Job	Run time
perf	33m 21s
documentation	1m 59s
benchcomp-tests	4m 40s
regression (macos-13)	28m 5s
regression (ubuntu-20.04)	23m 32s
write-json-symtab-regression	14m 43s
regression (ubuntu-22.04)	24m 28s
regression (macos-14)	20m 56s
	2h 31m 44s

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