Suggest if let on let refutable binding

src/test/ui/issues/issue-31561.stderr

@@ -12,6 +12,13 @@ LL | | }
 ...
 LL |       let Thing::Foo(y) = Thing::Foo(1);
    |           ^^^^^^^^^^^^^ patterns `Bar` and `Baz` not covered
+   |
+   = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
+   = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let Thing::Foo(y) = Thing::Foo(1) { /* */ }
+   |
 
 error: aborting due to previous error