libstdc++: [_GLIBCXX_DEBUG] Remove useless constructor checks

libstdc++: [_GLIBCXX_DEBUG] Remove useless constructor checks

Creating a safe iterator from a normal iterator is done within the library where we
already know that it is done correctly. The rare situation where a user would use safe
iterators for his own purpose is non-Standard code so outside _GLIBCXX_DEBUG scope. For
those reasons the __msg_init_singular is useless and can be removed.

Additionally in the copy constructor used for post-increment and post-decrement operators
the __msg_init_copy_singular check can also be ommitted because of the preliminary
__msg_bad_incr and __msg_bad_decr checks.

libstdc++-v3/ChangeLog:

	* include/debug/safe_iterator.h (_Safe_iterator<>::_Unchecked): New.
	(_Safe_iterator(const _Safe_iterator&, _Unchecked)): New.
	(_Safe_iterator::operator++(int)): Use latter.
	(_Safe_iterator::operator--(int)): Likewise.
	(_Safe_iterator(_Iterator, const _Safe_sequence_base*)): Remove !_M_insular()
	check.
	* include/debug/safe_local_iterator.h (_Safe_local_iterator<>::_Unchecked):
	New.
	(_Safe_local_iterator(const _Safe_local_iterator&, _Unchecked)): New.
	(_Safe_local_iterator::operator++(int)): Use latter.
	* src/c++11/debug.cc (_S_debug_messages): Add as comment the _Debug_msg_id
	entry associated to the array entry.