Avoid default-initializing auto_vec<T, N> storage, fix vec<vl_embed>

The following avoids default-initializing auto_vec storage for
non-POD T since that's not what the allocated storage fallback
will do and it's also not expected for existing cases like

  auto_vec<std::pair<unsigned, unsigned>, 64> elts;

which exist to optimize the allocation.

It also fixes the array accesses done by vec<vl_embed> to not
use its own m_vecdata member but instead access the container
provided storage via pointer arithmetic.

	* vec.h (vec<T, A, vl_embed>::m_vecdata): Remove.
	(vec<T, A, vl_embed>::m_vecpfx): Align as T to avoid
	changing alignment of vec<T, A, vl_embed> and simplifying
	address.
	(vec<T, A, vl_embed>::address): Compute as this + 1.
	(vec<T, A, vl_embed>::embedded_size): Use sizeof the
	vector instead of the offset of the m_vecdata member.
	(auto_vec<T, N>::m_data): Turn storage into
	uninitialized unsigned char.
	(auto_vec<T, N>::auto_vec): Allow allocation of one
	stack member.  Initialize m_vec in a special way to
	avoid later stringop overflow diagnostics.
	* vec.cc (test_auto_alias): New.
	(vec_cc_tests): Call it.