Remove from_fn lang item

It was probably a leftover from the old `?` desugaring but anyways, it's
unused now except for clippy, which can just use a diagnostics item.

clippy_lints/src/operators/cmp_owned.rs

@@ -49,10 +49,10 @@ fn check_op(cx: &LateContext<'_>, expr: &Expr<'_>, other: &Expr<'_>, left: bool)
             (arg, arg.span)
         },
         ExprKind::Call(path, [arg])
-            if path_def_id(cx, path).map_or(false, |id| {
-                if match_def_path(cx, id, &paths::FROM_STR_METHOD) {
+            if path_def_id(cx, path).map_or(false, |did| {
+                if match_def_path(cx, did, &paths::FROM_STR_METHOD) {
                     true
-                } else if cx.tcx.lang_items().from_fn() == Some(id) {
+                } else if cx.tcx.is_diagnostic_item(sym::from_fn, did) {
                     !is_copy(cx, typeck.expr_ty(expr))
                 } else {
                     false

clippy_lints/src/unnecessary_owned_empty_strings.rs

@@ -7,6 +7,7 @@
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::symbol::sym;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -54,7 +55,7 @@ fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
                         );
                 } else {
                     if_chain! {
-                        if Some(fun_def_id) == cx.tcx.lang_items().from_fn();
+                        if cx.tcx.is_diagnostic_item(sym::from_fn, fun_def_id);
                         if let [.., last_arg] = args;
                         if let ExprKind::Lit(spanned) = &last_arg.kind;
                         if let LitKind::Str(symbol, _) = spanned.node;

clippy_lints/src/useless_conversion.rs

@@ -161,7 +161,7 @@ fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
                         }
 
                         if_chain! {
-                            if Some(def_id) == cx.tcx.lang_items().from_fn();
+                            if cx.tcx.is_diagnostic_item(sym::from_fn, def_id);
                             if same_type_and_consts(a, b);
 
                             then {