Make missing argument placeholder more obvious that it's a placeholder

src/test/ui/error-codes/E0057.stderr

@@ -11,8 +11,8 @@ LL |     let f = |x| x * 3;
    |             ^^^
 help: provide the argument
    |
-LL |     let a = f({_});
-   |             ~~~~~~
+LL |     let a = f(/* value */);
+   |             ~~~~~~~~~~~~~~
 
 error[E0057]: this function takes 1 argument but 2 arguments were supplied
   --> $DIR/E0057.rs:5:13

src/test/ui/error-codes/E0060.stderr

@@ -11,8 +11,8 @@ LL |     fn printf(_: *const u8, ...) -> u32;
    |        ^^^^^^
 help: provide the argument
    |
-LL |     unsafe { printf({*const u8}); }
-   |              ~~~~~~~~~~~~~~~~~~~
+LL |     unsafe { printf(/* *const u8 */); }
+   |              ~~~~~~~~~~~~~~~~~~~~~~~
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0061.stderr

@@ -11,8 +11,8 @@ LL | fn f(a: u16, b: &str) {}
    |    ^ ------  -------
 help: provide the argument
    |
-LL |     f(0, {&str});
-   |     ~~~~~~~~~~~~
+LL |     f(0, /* &str */);
+   |     ~~~~~~~~~~~~~~~~
 
 error[E0061]: this function takes 1 argument but 0 arguments were supplied
   --> $DIR/E0061.rs:9:5
@@ -27,8 +27,8 @@ LL | fn f2(a: u16) {}
    |    ^^ ------
 help: provide the argument
    |
-LL |     f2({u16});
-   |     ~~~~~~~~~
+LL |     f2(/* u16 */);
+   |     ~~~~~~~~~~~~~
 
 error: aborting due to 2 previous errors