[alias] unsafe casts: be more explicit about unsoundness

src/plugins/alias/src/simplified.ml

src/plugins/alias/tests/basic/oracle/cast1.res.oracle

src/plugins/alias/tests/fixed_bugs/oracle/origin.res.oracle

...ins/alias/tests/fixed_bugs/oracle/origin_simpl.res.oracle

.../alias/tests/fixed_bugs/oracle/reduce_by_valid.res.oracle

src/plugins/alias/tests/fixed_bugs/oracle/semver.res.oracle

src/plugins/alias/tests/fixed_bugs/oracle/tkn-2.res.oracle

...ugins/alias/tests/fixed_bugs/oracle/union_vmap.res.oracle

src/plugins/alias/tests/offsets/oracle/nested1.res.oracle

src/plugins/alias/tests/offsets/oracle/nested2.res.oracle

src/plugins/alias/tests/offsets/oracle/structure1.res.oracle

src/plugins/alias/tests/offsets/oracle/structure2.res.oracle

src/plugins/alias/tests/offsets/oracle/structure4.res.oracle

src/plugins/alias/tests/offsets/oracle/structure5.res.oracle