Use proper span for tuple index parsed as float

Fix diagnostic suggestion from:

```rust
help: try parenthesizing the first index
  |     (1, (2, 3)).((1, (2, 3)).1).1;
```

to the correct:

```rust
help: try parenthesizing the first index
  |     ((1, (2, 3)).1).1;
```

src/test/ui/suggestions/tuple-float-index.rs

@@ -11,7 +11,5 @@
 // compile-flags: -Z parse-only
 
 fn main () {
-    (1, (2, 3)).1.1; //~ ERROR unexpected token
-                     //~^ HELP try parenthesizing the first index
-                     //~| SUGGESTION ((1, (2, 3)).1).1
+    (1, (2, 3)).1.1;
 }

src/test/ui/suggestions/tuple-float-index.stderr

@@ -0,0 +1,11 @@
+error: unexpected token: `1.1`
+  --> $DIR/tuple-float-index.rs:14:17
+   |
+14 |     (1, (2, 3)).1.1;
+   |                 ^^^ unexpected token
+   |
+help: try parenthesizing the first index
+   |     ((1, (2, 3)).1).1;
+
+error: aborting due to previous error
+