Remove where bound from clean_fn_decl_with_args

Basically, this entails moving the arguments cleaning to the call site.

I extracted several local variables because:

1. It makes the code easier to read and understand.

2. If I hadn't, the extra `clean()` calls would have caused complicated
   tuples to be split across several lines.

3. I couldn't just extract local variables for `args` because then the
   arguments would be cleaned *before* the generics, while rustdoc expects
   them to be cleaned *after*. Only extracting `args` caused panics like
   this:

       thread 'rustc' panicked at 'assertion failed: cx.impl_trait_bounds.is_empty()',
       src/librustdoc/clean/utils.rs:462:5

   Extracting variables makes the control flow -- and the required
   order of cleaning -- more explicit.

Author: camelid

src/librustdoc/clean/mod.rs

@@ -762,7 +762,10 @@ fn clean_fn_or_proc_macro(
 impl<'a> Clean<Function> for (&'a hir::FnSig<'a>, &'a hir::Generics<'a>, hir::BodyId) {
     fn clean(&self, cx: &mut DocContext<'_>) -> Function {
         let (generics, decl) = enter_impl_trait(cx, |cx| {
-            (self.1.clean(cx), clean_fn_decl_with_args(cx, &*self.0.decl, self.2))
+            let generics = self.1.clean(cx);
+            let args = (self.0.decl.inputs, self.2).clean(cx);
+            let decl = clean_fn_decl_with_args(cx, &*self.0.decl, args);
+            (generics, decl)
         });
         Function { decl, generics, header: self.0.header }
     }
@@ -805,19 +808,12 @@ impl<'a> Clean<Arguments> for (&'a [hir::Ty<'a>], hir::BodyId) {
     }
 }
 
-fn clean_fn_decl_with_args<'a, A: Copy>(
+fn clean_fn_decl_with_args(
     cx: &mut DocContext<'_>,
-    decl: &'a hir::FnDecl<'a>,
-    args: A,
-) -> FnDecl
-where
-    (&'a [hir::Ty<'a>], A): Clean<Arguments>,
-{
-    FnDecl {
-        inputs: (decl.inputs, args).clean(cx),
-        output: decl.output.clean(cx),
-        c_variadic: decl.c_variadic,
-    }
+    decl: &hir::FnDecl<'_>,
+    args: Arguments,
+) -> FnDecl {
+    FnDecl { inputs: args, output: decl.output.clean(cx), c_variadic: decl.c_variadic }
 }
 
 impl<'tcx> Clean<FnDecl> for (DefId, ty::PolyFnSig<'tcx>) {
@@ -897,7 +893,10 @@ impl Clean<Item> for hir::TraitItem<'_> {
                 }
                 hir::TraitItemKind::Fn(ref sig, hir::TraitFn::Required(names)) => {
                     let (generics, decl) = enter_impl_trait(cx, |cx| {
-                        (self.generics.clean(cx), clean_fn_decl_with_args(cx, sig.decl, names))
+                        let generics = self.generics.clean(cx);
+                        let args = (sig.decl.inputs, names).clean(cx);
+                        let decl = clean_fn_decl_with_args(cx, sig.decl, args);
+                        (generics, decl)
                     });
                     let mut t = Function { header: sig.header, decl, generics };
                     if t.header.constness == hir::Constness::Const
@@ -1731,7 +1730,8 @@ impl Clean<BareFunctionDecl> for hir::BareFnTy<'_> {
     fn clean(&self, cx: &mut DocContext<'_>) -> BareFunctionDecl {
         let (generic_params, decl) = enter_impl_trait(cx, |cx| {
             let generic_params = self.generic_params.iter().map(|x| x.clean(cx)).collect();
-            let decl = clean_fn_decl_with_args(cx, self.decl, self.param_names);
+            let args = (self.decl.inputs, self.param_names).clean(cx);
+            let decl = clean_fn_decl_with_args(cx, self.decl, args);
             (generic_params, decl)
         });
         BareFunctionDecl { unsafety: self.unsafety, abi: self.abi, decl, generic_params }
@@ -2029,7 +2029,10 @@ impl Clean<Item> for (&hir::ForeignItem<'_>, Option<Symbol>) {
                 hir::ForeignItemKind::Fn(decl, names, ref generics) => {
                     let abi = cx.tcx.hir().get_foreign_abi(item.hir_id());
                     let (generics, decl) = enter_impl_trait(cx, |cx| {
-                        (generics.clean(cx), clean_fn_decl_with_args(cx, decl, names))
+                        let generics = generics.clean(cx);
+                        let args = (decl.inputs, names).clean(cx);
+                        let decl = clean_fn_decl_with_args(cx, decl, args);
+                        (generics, decl)
                     });
                     ForeignFunctionItem(Function {
                         decl,