Fix Issue 10460 - auto ref for 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.
Merge pull request #2461 from fb-pja/bug10460

Fix Issue 10460 - auto ref Union, SetDifference, SetSymmetricDifference