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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184603 91177308-0d34-0410-b5e6-96231b3b80d8