Extend the C++03 definition of POD to include defaulted functions

The AST/conditionally-trivial-smfs tests look a bit questionable, but
are consistent with GCC's POD-ness, at least as far as packing is
concerned: https://godbolt.org/z/36nqPMbKM
(questionable because it looks like the type would be non-copyable, so
how could it be pod? But the calling convention/pass by value seems to
work correctly (local testing verifies that this behavior is preserved
even with this patch: https://godbolt.org/z/3Pa89zsv6 ))

Differential Revision: https://reviews.llvm.org/D119051