vec.h: Properly destruct elements in auto_vec auto storage [PR118400]

For T with non-trivial destructors, we were destructing objects in the
vector on release only when not using auto storage of auto_vec.

The following patch calls truncate (0) instead of m_vecpfx.m_num clearing,
and truncate takes care of that destruction:
  unsigned l = length ();
  gcc_checking_assert (l >= size);
  if (!std::is_trivially_destructible <T>::value)
    vec_destruct (address () + size, l - size);
  m_vecpfx.m_num = size;

2025-01-16  Jakub Jelinek  <jakub@redhat.com>

	PR ipa/118400
	* vec.h (vec<T, va_heap, vl_ptr>::release): Call m_vec->truncate (0)
	instead of clearing m_vec->m_vecpfx.m_num.