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

@@ -0,0 +1,15 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z parse-only
+
+fn main () {
+    (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
+