implement initial codegen for aggregate return functions. This implements

implement initial codegen for aggregate return functions.  This implements
codegen for:

_Complex double bar(int);
void test(_Complex double*);

void test2(int c) {
  _Complex double X;
  X = bar(1);
  test(&X);
}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@40993 91177308-0d34-0410-b5e6-96231b3b80d8