safe transmute: use Assume struct to provide analysis options

This was left as a TODO in #92268, and brings the trait more in
line with what was defined in MCP411.

`Assume::visibility` has been renamed to `Assume::safety`, as
library safety is what's actually being assumed; visibility is
just the mechanism by which it is currently checked (this may
change).

ref: https://github.com/rust-lang/compiler-team/issues/411
ref: https://github.com/rust-lang/rust/issues/99571

compiler/rustc_transmute/src/maybe_transmutable/tests.rs

@@ -13,7 +13,7 @@ mod bool {
             layout::Tree::<Def, !>::bool(),
             layout::Tree::<Def, !>::bool(),
             (),
-            crate::Assume { alignment: false, lifetimes: false, validity: true, visibility: false },
+            crate::Assume { alignment: false, lifetimes: false, validity: true, safety: false },
             UltraMinimal,
         )
         .answer();
@@ -26,7 +26,7 @@ mod bool {
             layout::Dfa::<!>::bool(),
             layout::Dfa::<!>::bool(),
             (),
-            crate::Assume { alignment: false, lifetimes: false, validity: true, visibility: false },
+            crate::Assume { alignment: false, lifetimes: false, validity: true, safety: false },
             UltraMinimal,
         )
         .answer();