libstdc++: Add hint to compiler about vector invariants [PR106434]

libstdc++: Add hint to compiler about vector invariants [PR106434]

The PR shows a bogus warning where jump threading generates code for the
undefined case that the insertion point is a value-initialized iterator
but _M_finish and _M_end_of_storage are unequal (so at least one must be
non-null). Using __builtin_unreachable() removes the bogus warning. Also
add an assertion to diagnose undefined misuses of a null iterator here,
so we don't just silently optimize that undefined code to something
unsafe.

libstdc++-v3/ChangeLog:

	PR c++/106434
	* include/bits/vector.tcc (insert(const_iterator, const T&)):
	Add assertion and optimization hint that the iterator for the
	insertion point must be non-null.