PR libstdc++/79254 fix exception-safety of std::string copy assignment

PR libstdc++/79254 fix exception-safety of std::string copy assignment

	PR libstdc++/79254
	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
	(basic_string::operator=(const basic_string&)): If source object is
	small just deallocate, otherwise perform new allocation before
	making any changes.
	* testsuite/21_strings/basic_string/allocator/wchar_t/copy_assign.cc:
	Test exception-safety of copy assignment when allocator propagates.
	* testsuite/21_strings/basic_string/allocator/char/copy_assign.cc:
	Likewise.
	* testsuite/util/testsuite_allocator.h (uneq_allocator::swap): Make
	std::swap visible.

From-SVN: r245088