[stdlib] Move empty check for string concatenation to front

[Nirma]

I think its best to check if the left hand side is empty first before making a mutable copy so I moved the empty check to the front of the function.

This way a mutable copy of lhs is made only when it is actually used.

[lattner]

Nice catch!

[gottesmm]

Seems like an obviously good change.

@swift-ci Please smoke test and merge

[Nirma]

@gottesmm The OS X Smoke test passed but the smoke test for the linux platform failed.
The failure seems unrelated to me but how does it look?

Thanks!!