libstdc++: Fix debug containers for C++98 mode

libstdc++: Fix debug containers for C++98 mode

Since r12-5072 made _Safe_container::operator=(const _Safe_container&)
protected, the debug containers no longer compile in C++98 mode. They
have user-provided copy assignment operators in C++98 mode, and they
assign each base class in turn. The 'this->_M_safe() = __x' expressions
fail, because calling a protected member function is only allowed via
'this'. They could be fixed by using this->_Safe::operator=(__x) but a
simpler solution is to just remove the user-provided assignment
operators and let the compiler define them (as we do for C++11 and
later, by defining them as defaulted).

The only change needed for that to work is to define the _Safe_vector
copy assignment operator in C++98 mode, so that the implicit
__gnu_debug::vector::operator= definition will call it, instead of
needing to call _M_update_guaranteed_capacity() manually.

libstdc++-v3/ChangeLog:

	* include/debug/deque (deque::operator=(const deque&)): Remove
	definition.
	* include/debug/list (list::operator=(const list&)): Likewise.
	* include/debug/map.h (map::operator=(const map&)): Likewise.
	* include/debug/multimap.h (multimap::operator=(const multimap&)):
	Likewise.
	* include/debug/multiset.h (multiset::operator=(const multiset&)):
	Likewise.
	* include/debug/set.h (set::operator=(const set&)): Likewise.
	* include/debug/string (basic_string::operator=(const basic_string&)):
	Likewise.
	* include/debug/vector (vector::operator=(const vector&)):
	Likewise.
	(_Safe_vector::operator=(const _Safe_vector&)): Define for
	C++98 as well.