Reachable symbols should be determined by the MIR

Currently, the set of reachable symbols is determined by the privacy, and whether or a function that references the symbol could be inlined cross-crate. With MIR inlining (see #36593), symbols that were previously considered unreachable from outside the crate may become reachable if an item that isn't inlined cross-crate is inlined into another function that _is_ inlined cross-crate.
