Fix a regression in reporting illegal C<return>.

The X::ControlFlow set of exceptions coming with the backtrace of the
place that did the wrong control flow is certainly a good thing, but
unfortunately the patch to make that happen meant we lost backtraces
for C<return>. This fixes the regression.

src/core/Exception.pm

@@ -1581,6 +1581,10 @@ my class X::ControlFlow is Exception {
     has $.enclosing; # ....  outside a loop
     has $.backtrace; # where the bougs control flow op was
 
+    method backtrace() {
+        $!backtrace || nextsame();
+    }
+
     method message() { "$.illegal without $.enclosing" }
 }
 my class X::ControlFlow::Return is X::ControlFlow {