PR libstdc++/84087 add default arguments to basic_string members (LWG 2268)

PR libstdc++/84087 add default arguments to basic_string members (LWG 2268)

This change was a DR against C++11 and so should have been implemented
years ago.

	PR libstdc++/84087 LWG DR 2268 basic_string default arguments
	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI=1]
	(append(const basic_string&, size_type, size_type)
	(assign(const basic_string&, size_type, size_type)
	(insert(size_type, const basic_string&, size_type, size_type)
	(replace(size_type,size_type,const basic_string&,size_type,size_type)
	(compare(size_type,size_type,constbasic_string&,size_type,size_type)):
	Add default arguments (LWG 2268).
	[_GLIBCXX_USE_CXX11_ABI=0]
	(append(const basic_string&, size_type, size_type)
	(assign(const basic_string&, size_type, size_type)
	(insert(size_type, const basic_string&, size_type, size_type)
	(replace(size_type,size_type,const basic_string&,size_type,size_type)
	(compare(size_type,size_type,constbasic_string&,size_type,size_type)):
	Likewise.
	* testsuite/21_strings/basic_string/dr2268.cc: New test.

From-SVN: r259895