PR libstdc++/79254 simplify exception-safety in copy assignment

PR libstdc++/79254 simplify exception-safety in copy assignment

	PR libstdc++/79254
	* config/abi/pre/gnu.ver: Remove recently added symbols.
	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
	(basic_string::_M_copy_assign): Remove.
	(basic_string::operator=(const basic_string&)): Don't dispatch to
	_M_copy_assign. If source object is small just deallocate, otherwise
	perform new allocation before making any changes.
	* include/bits/basic_string.tcc [_GLIBCXX_USE_CXX11_ABI]
	(basic_string::_M_copy_assign(const basic_string&, true_type)):
	Remove.
	* testsuite/21_strings/basic_string/allocator/char/copy_assign.cc:
	Test cases where the allocators are equal or the string is small.
	* testsuite/21_strings/basic_string/allocator/wchar_t/copy_assign.cc:
	Likewise.

From-SVN: r245085