Parse async fn header.

This is gated on edition 2018 & the `async_await` feature gate.

The parser will accept `async fn` and `async unsafe fn` as fn
items. Along the same lines as `const fn`, only `async unsafe fn`
is permitted, not `unsafe async fn`.The parser will not accept
`async` functions as trait methods.

To do a little code clean up, four fields of the function type
struct have been merged into the new `FnHeader` struct: constness,
asyncness, unsafety, and ABI.

Also, a small bug in HIR printing is fixed: it previously printed
`const unsafe fn` as `unsafe const fn`, which is grammatically
incorrect.

src/libsyntax/fold.rs

@@ -881,11 +881,11 @@ pub fn noop_fold_item_kind<T: Folder>(i: ItemKind, folder: &mut T) -> ItemKind {
         ItemKind::Const(t, e) => {
             ItemKind::Const(folder.fold_ty(t), folder.fold_expr(e))
         }
-        ItemKind::Fn(decl, unsafety, constness, abi, generics, body) => {
+        ItemKind::Fn(decl, header, generics, body) => {
             let generics = folder.fold_generics(generics);
             let decl = folder.fold_fn_decl(decl);
             let body = folder.fold_block(body);
-            ItemKind::Fn(decl, unsafety, constness, abi, generics, body)
+            ItemKind::Fn(decl, header, generics, body)
         }
         ItemKind::Mod(m) => ItemKind::Mod(folder.fold_mod(m)),
         ItemKind::ForeignMod(nm) => ItemKind::ForeignMod(folder.fold_foreign_mod(nm)),
@@ -1082,9 +1082,7 @@ pub fn noop_fold_foreign_item_simple<T: Folder>(ni: ForeignItem, folder: &mut T)
 
 pub fn noop_fold_method_sig<T: Folder>(sig: MethodSig, folder: &mut T) -> MethodSig {
     MethodSig {
-        abi: sig.abi,
-        unsafety: sig.unsafety,
-        constness: sig.constness,
+        header: sig.header,
         decl: folder.fold_fn_decl(sig.decl)
     }
 }