Tweak format string error to point at arguments always

Add secondary span labels with no text to make it clear when there's a
mismatch bewteen the positional arguments in a format string and the
arguments to the macro. This shouldn't affect experienced users, but it
should make it easier for newcomers to more clearly understand how
`format!()` and `println!()` are supposed to be used.

```
error: 2 positional arguments in format string, but there is 1 argument
 --> file8.rs:2:14
  |
2 |     format!("{} {}", 1);
  |              ^^ ^^   -
```

instead of

```
error: 2 positional arguments in format string, but there is 1 argument
 --> file8.rs:2:14
  |
2 |     format!("{} {}", 1);
  |              ^^ ^^
```

src/libsyntax_ext/format.rs

@@ -278,7 +278,7 @@ impl<'a, 'b> Context<'a, 'b> {
     /// format string.
     fn report_invalid_references(&self, numbered_position_args: bool) {
         let mut e;
-        let sp = if self.is_literal {
+        let sp = if self.is_literal { // Point at the formatting arguments.
             MultiSpan::from_spans(self.arg_spans.clone())
         } else {
             MultiSpan::from_span(self.fmtsp)
@@ -304,6 +304,9 @@ impl<'a, 'b> Context<'a, 'b> {
                     self.describe_num_args(),
                 ),
             );
+            for arg in &self.args { // Point at the arguments that will be formatted.
+                e.span_label(arg.span, "");
+            }
         } else {
             let (mut refs, spans): (Vec<_>, Vec<_>) = refs.unzip();
             // Avoid `invalid reference to positional arguments 7 and 7 (there is 1 argument)`

src/test/ui/fmt/format-string-error.rs

@@ -48,4 +48,6 @@ fn main() {
 
 "###);
     //~^^^ ERROR invalid format string: unmatched `}` found
+    println!("{} {} {}", 1, 2);
+    //~^ ERROR 3 positional arguments in format string, but there are 2 arguments
 }

src/test/ui/fmt/format-string-error.stderr

@@ -107,5 +107,11 @@ LL |         }
    |
    = note: if you intended to print `}`, you can escape it using `}}`
 
-error: aborting due to 12 previous errors
+error: 3 positional arguments in format string, but there are 2 arguments
+  --> $DIR/format-string-error.rs:51:15
+   |
+LL |     println!("{} {} {}", 1, 2);
+   |               ^^ ^^ ^^   -  -
+
+error: aborting due to 13 previous errors
 

src/test/ui/if/ifmt-bad-arg.stderr

@@ -224,8 +224,9 @@ error: 4 positional arguments in format string, but there are 3 arguments
   --> $DIR/ifmt-bad-arg.rs:78:15
    |
 LL |     println!("{} {:.*} {}", 1, 3.2, 4);
-   |               ^^ ^^--^ ^^      --- this parameter corresponds to the precision flag
-   |                    |
+   |               ^^ ^^--^ ^^   -  ---  -
+   |                    |           |
+   |                    |           this parameter corresponds to the precision flag
    |                    this precision flag adds an extra required argument at position 1, which is why there are 4 arguments expected
    |
    = note: positional arguments are zero-based
@@ -235,8 +236,9 @@ error: 4 positional arguments in format string, but there are 3 arguments
   --> $DIR/ifmt-bad-arg.rs:81:15
    |
 LL |     println!("{} {:07$.*} {}", 1, 3.2, 4);
-   |               ^^ ^^^----^ ^^      --- this parameter corresponds to the precision flag
-   |                     | |
+   |               ^^ ^^^----^ ^^   -  ---  -
+   |                     | |           |
+   |                     | |           this parameter corresponds to the precision flag
    |                     | this precision flag adds an extra required argument at position 1, which is why there are 4 arguments expected
    |                     this width flag expects an `usize` argument at position 7, but there are 3 arguments
    |