libstdc++: Avoid stack overflow in std::vector (PR 94540)

libstdc++: Avoid stack overflow in std::vector (PR 94540)

The std::__uninitialized_default_n algorithm used by std::vector creates
an initial object as a local variable then copies that into the
destination range. If the object is too large for the stack this
crashes. We should create the first object directly into the
destination and then copy it from there.

This doesn't fix the bug for C++98, because in that case the initial
value is created as a default argument of the vector constructor i.e. in
the user's code, not inside libstdc++. We can't prevent that.

	PR libstdc++/94540
	* include/bits/stl_uninitialized.h (__uninitialized_default_1<true>):
	Construct the first value at *__first instead of on the stack.
	(__uninitialized_default_n_1<true>): Likewise.
	Improve comments on several of the non-standard algorithms.
	* testsuite/20_util/specialized_algorithms/uninitialized_default/94540.cc:
	New test.
	* testsuite/20_util/specialized_algorithms/uninitialized_default_n/94540.cc:
	New test.
	* testsuite/20_util/specialized_algorithms/uninitialized_value_construct/94540.cc:
	New test.
	* testsuite/20_util/specialized_algorithms/uninitialized_value_construct_n/94540.cc:
	New test.
	* testsuite/23_containers/vector/cons/94540.cc: New test.