Add regression test for float literal tuple indices

Add a new regression test in order to highlight the fix for Rust-GCC#2659.

gcc/testsuite/ChangeLog:

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

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

gcc/testsuite/rust/compile/not_a_float_literal_tuple_index.rs

@@ -0,0 +1,3 @@
+fn main() {
+    (1, (2, 3)).1.1;
+}