Add a new test for float disambiguation

This new regression test highlight the behavior fixed for float
disambiguation with empty floating point.

gcc/testsuite/ChangeLog:

	* rust/compile/tuple_float_index.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

gcc/testsuite/rust/compile/tuple_float_index.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let tuple = (((),()),);
+
+    // Do not reformat, the space after the second dot is required
+    let _ = tuple.0. 1;
+}