Check if let expr usage in manual_flatten

`manual_flatten` should not trigger when match expression in `if let` is
going to be used.

clippy_lints/src/loops/manual_flatten.rs

@@ -1,6 +1,7 @@
 use super::utils::make_iterator_snippet;
 use super::MANUAL_FLATTEN;
 use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::visitors::LocalUsedVisitor;
 use clippy_utils::{is_lang_ctor, path_to_local_id};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
@@ -47,6 +48,9 @@ pub(super) fn check<'tcx>(
             let some_ctor = is_lang_ctor(cx, qpath, OptionSome);
             let ok_ctor = is_lang_ctor(cx, qpath, ResultOk);
             if some_ctor || ok_ctor;
+            // Ensure epxr in `if let` is not used afterwards
+            let mut used_visitor = LocalUsedVisitor::new(cx, pat_hir_id);
+            if !match_arms.iter().any(|arm| used_visitor.check_arm(arm));
             then {
                 let if_let_type = if some_ctor { "Some" } else { "Ok" };
                 // Prepare the error message