Fix Issue 10460 - auto ref Union, SetDifference, SetSymmetricDifference

https://issues.dlang.org/show_bug.cgi?id=10460

The rearrangement of SetSymmetricDifference.front was needed since auto ref doesn't deduce correctly when there are multiple return statements (by design), so a single return statement with the ternary operator was used.
This makes the range assignable. Will the range sill play nice if you change the front to be the sentinel (I forget if you stop before or at the sentinel)? And if yes, could you add a test for it?