Check for trivial constructibility before emptiness in ARM ABI.

According to the Itanium ABI (3.1.1), types with non-trivial copy constructors
passed by value should be passed indirectly, with the caller creating a
temporary.

We got this mostly correct, but forgot that empty structs can have non-trivial
constructors too and passed them incorrectly. This simply reverses the order of
the check.