modified lint to work with MIR

Working with MIR let's us exclude expressions like `&fn_name as &dyn Something`
and `(&fn_name)()`. Also added ABI, unsafety and whether a function is variadic
in the lint suggestion, included the `&` in the span of the lint and updated the
test.

compiler/rustc_mir/src/transform/mod.rs

@@ -27,6 +27,7 @@ pub mod dest_prop;
 pub mod dump_mir;
 pub mod early_otherwise_branch;
 pub mod elaborate_drops;
+pub mod function_references;
 pub mod generator;
 pub mod inline;
 pub mod instcombine;
@@ -266,6 +267,7 @@ fn mir_const<'tcx>(
             // MIR-level lints.
             &check_packed_ref::CheckPackedRef,
             &check_const_item_mutation::CheckConstItemMutation,
+            &function_references::FunctionReferences,
             // What we need to do constant evaluation.
             &simplify::SimplifyCfg::new("initial"),
             &rustc_peek::SanityCheck,