Add test for using $:literal to index a tuple

Currently fails to build:

    error: unexpected token: `0`
      --> src/test/ui/macros/macro-interpolation.rs:19:16
       |
    LL |         $var . $field
       |                ^^^^^^
    ...
    LL |     let _ = field!(tuple.0);
       |             --------------- in this macro invocation
       |
       = note: this error originates in the macro `field` (in Nightly builds, run with -Z macro-backtrace for more info)

    error: macro expansion ignores token `0` and any following
      --> src/test/ui/macros/macro-interpolation.rs:19:16
       |
    LL |         $var . $field
       |                ^^^^^^
    ...
    LL |     let _ = field!(tuple.0);
       |             --------------- caused by the macro expansion here
       |
       = note: the usage of `field!` is likely invalid in expression context

    error: aborting due to 2 previous errors

Author: dtolnay

src/test/ui/macros/macro-interpolation.rs

@@ -24,10 +24,19 @@ macro_rules! qpath {
     };
 }
 
+macro_rules! field {
+    ($var:ident . $field:literal) => {
+        $var . $field
+    };
+}
+
 pub fn main() {
     let _: qpath!(path, <str as ToOwned>::Owned);
     let _: qpath!(ty, <str as ToOwned>::Owned);
 
+    let tuple = ('x',);
+    let _ = field!(tuple.0);
+
     assert!(overly_complicated!(f, x, Option<usize>, { return Some(x); },
                                Some(8), Some(y), y) == 8)
 }