[SystemZ ABI] Allow class types in GetSingleElementType

The SystemZ ABI specifies that aggregate types with just a single
member of floating-point type shall be passed as if they were just
a scalar of that type.  This applies to both struct and class types
(but not unions).

However, the current ABI support code in clang only checks this
case for struct types, which means that for class types, generated
code does not adhere to the platform ABI.

Fixed by accepting both struct and class types in the
SystemZABIInfo::GetSingleElementType routine.