libstdc++: Fix std::basic_stacktrace special members [PR105031]

libstdc++: Fix std::basic_stacktrace special members [PR105031]

The PR points out that there is a non-constant condition used for an
if-constexpr statement, but there are several other problems with the
copy, move and swap members of std::basic_stacktrace.

libstdc++-v3/ChangeLog:

	PR libstdc++/105031
	* include/std/stacktrace (basic_stacktrace::basic_stacktrace):
	Fix allocator usage in constructors.
	(basic_stacktrace::operator=(const basic_stacktrace&)): Do not
	try to reallocate using const allocator.
	(basic_stacktrace::operator=(basic_stacktrace&&)): Fix
	if-constexpr with non-constant condition. Do not allocate new
	storage if allocator propagates. Do not set _M_size if
	allocation fails.
	(basic_stacktrace::swap(basic_stacktrace&)): Fix typo. Add
	assertion that non-propagating allocators are equal.
	* testsuite/19_diagnostics/stacktrace/stacktrace.cc: New test.