Fix testsuite failures for __gnu_debug::string with old ABI

Fix testsuite failures for __gnu_debug::string with old ABI

The __gnu_debug string (mostly) implements the C++11 API, but when it
wraps the old COW string many of the member functions in the base class
have the wrong parameter types or return types. This makes the
__gnu_debug::string type adapt itself to the base class API. This
actually makes the debug string slightly more conforming than the
underlying string type when using the old ABI.

	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
	(basic_string::__const_iterator): Change access to protected.
	[!_GLIBCXX_USE_CXX11_ABI] (basic_string::__const_iterator): Define
	as typedef for iterator.
	* include/debug/string (__const_iterator): Use typedef from base.
	(insert(const_iterator, _CharT))
	(replace(const_iterator, const_iterator, const basic_string&))
	(replace(const_iterator, const_iterator, const _CharT*, size_type))
	(replace(const_iterator, const_iterator, const CharT*))
	(replace(const_iterator, const_iterator, size_type, _CharT))
	(replace(const_iterator, const_iterator, _InputIter, _InputIter))
	(replace(const_iterator, const_iterator, initializer_list<_CharT>)):
	Change const_iterator parameters to __const_iterator.
	(insert(iterator, size_type, _CharT)): Add C++98 overload.
	(insert(const_iterator, _InputIterator, _InputIterator)): Change
	const_iterator parameter to __const_iterator.
	[!_GLIBCXX_USE_CXX11_ABI]: Add workaround for incorrect return type
	of base's member function.
	(insert(const_iterator, size_type, _CharT)) [!_GLIBCXX_USE_CXX11_ABI]:
	Likewise.
	(insert(const_iterator, initializer_list<_CharT>))
	[!_GLIBCXX_USE_CXX11_ABI]: Likewise.
	* testsuite/21_strings/basic_string/init-list.cc: Remove effective
	target directive.

From-SVN: r263808