Use JointHidden in a couple of suitable places.

This improves the `INPUT` output of one test. The `DEEP-RE-COLLECTED`
output is unchanged because the `JointHidden` gets converted to `Alone`
when it goes in and out of the proc macro.

compiler/rustc_ast/src/tokenstream.rs

@@ -661,11 +661,11 @@ impl TokenStream {
             if attr_style == AttrStyle::Inner {
                 vec![
                     TokenTree::token_joint(token::Pound, span),
-                    TokenTree::token_alone(token::Not, span),
+                    TokenTree::token_joint_hidden(token::Not, span),
                     body,
                 ]
             } else {
-                vec![TokenTree::token_alone(token::Pound, span), body]
+                vec![TokenTree::token_joint_hidden(token::Pound, span), body]
             }
         }
     }

tests/ui/proc-macro/issue-78675-captured-inner-attrs.stdout

@@ -1,4 +1,5 @@
-PRINT-BANG INPUT (DISPLAY): foo! { #[fake_attr] mod bar { #! [doc = r" Foo"] } }
+PRINT-BANG INPUT (DISPLAY): foo! { #[fake_attr] mod bar { #![doc = r" Foo"] } }
+PRINT-BANG DEEP-RE-COLLECTED (DISPLAY): foo! { #[fake_attr] mod bar { #! [doc = r" Foo"] } }
 PRINT-BANG INPUT (DEBUG): TokenStream [
     Ident {
         ident: "foo",