libstdc++: [_GLIBCXX_DEBUG] Remove _Safe_container<>::_M_safe()

     Container code cleanup to get rid of _Safe_container<>::_M_safe() 
and just
     _Safe:: calls which use normal inheritance. Also remove several 
usages of _M_base()
     which can be most of the time ommitted and sometimes replace with 
explicit _Base::
     calls.

     libstdc++-v3/ChangeLog:

             * include/debug/safe_container.h 
(_Safe_container<>::_M_safe): Remove.
             * include/debug/deque 
(deque::operator=(initializer_list<>)): Replace
             _M_base() call with _Base:: call.
             (deque::operator[](size_type)): Likewise.
             * include/debug/forward_list (forward_list(forward_list&&, 
const allocator_type&):
             Remove _M_safe() and _M_base() calls.
             (forward_list::operator=(initializer_list<>)): Remove 
_M_base() calls.
             (forward_list::splice_after, forward_list::merge): Likewise.
             * include/debug/list (list(list&&, const allocator_type&)):
             Remove _M_safe() and _M_base() calls.
             (list::operator=(initializer_list<>)): Remove _M_base() calls.
             (list::splice, list::merge): Likewise.
             * include/debug/map.h (map(map&&, const allocator_type&)):
             Remove _M_safe() and _M_base() calls.
             (map::operator=(initializer_list<>)): Remove _M_base() calls.
             * include/debug/multimap.h (multimap(multimap&&, const 
allocator_type&)):
             Remove _M_safe() and _M_base() calls.
             (multimap::operator=(initializer_list<>)): Remove _M_base() 
calls.
             * include/debug/set.h (set(set&&, const allocator_type&)):
             Remove _M_safe() and _M_base() calls.
             (set::operator=(initializer_list<>)): Remove _M_base() calls.
             * include/debug/multiset.h (multiset(multiset&&, const 
allocator_type&)):
             Remove _M_safe() and _M_base() calls.
             (multiset::operator=(initializer_list<>)): Remove _M_base() 
calls.
             * include/debug/string (basic_string(basic_string&&, const 
allocator_type&)):
             Remove _M_safe() and _M_base() calls.
             (basic_string::operator=(initializer_list<>)): Remove 
_M_base() call.
             (basic_string::operator=(const _CharT*), 
basic_string::operator=(_CharT)): Likewise.
             (basic_string::operator[](size_type), 
basic_string::operator+=(const basic_string&)): Likewise.
             (basic_string::operator+=(const _Char*), 
basic_string::operator+=(_CharT)): Likewise.
             * include/debug/unordered_map 
(unordered_map(unordered_map&&, const allocator_type&)):
             Remove _M_safe() and _M_base() calls.
             (unordered_map::operator=(initializer_list<>), 
unordered_map::merge): Remove _M_base() calls.
             (unordered_multimap(unordered_multimap&&, const 
allocator_type&)):
             Remove _M_safe() and _M_base() calls.
(unordered_multimap::operator=(initializer_list<>), 
unordered_multimap::merge):
             Remove _M_base() calls.
             * include/debug/unordered_set 
(unordered_set(unordered_set&&, const allocator_type&)):
             Remove _M_safe() and _M_base() calls.
             (unordered_set::operator=(initializer_list<>), 
unordered_set::merge): Remove _M_base() calls.
             (unordered_multiset(unordered_multiset&&, const 
allocator_type&)):
             Remove _M_safe() and _M_base() calls.
(unordered_multiset::operator=(initializer_list<>), 
unordered_multiset::merge):
             Remove _M_base() calls.
             * include/debug/vector (vector(vector&&, const 
allocator_type&):
             Remove _M_safe() and _M_base() calls.
             (vector::operator=(initializer_list<>)): Remove _M_base() 
calls.
             (vector::operator[](size_type)): Likewise.

Tested under Linux x86_64 _GLIBCXX_DEBUG mode -std=gnu++14 and -std=gnu++98.

Ok to commit ?

François