Re-enable test with modifications for Neon vector changes in clang.

I mistakenly thought that this was checking for vector name mangling, but
it is not.  Since we're no longer wrapping Neon vectors in structs, this
test can just return a vector directly.  There are already other tests for
that, so just to make this interesting, change the test to return a struct
of two vectors.

llvm-svn: 119434