Add allocator-extended copy/move ctors to COW string

Add allocator-extended copy/move ctors to COW string

Add these constructors from C++11 which were missing from the COW
basic_string. Additionally simplify the definitions of the
basic_string::reference and basic_string::const_reference types as
required by C++11.

This allows filesystem::path::string<Allocator>() to be simplified, so
that the same code is used for both basic_string implementations.

	* config/abi/pre/gnu.ver (GLIBCXX_3.4.26): Export allocator-extended
	copy/move constructors for old std::basic_string.
	* include/bits/basic_string.h [!_GLIBCXX_USE_CXX11_ABI]
	(basic_string::reference, basic_string::const_reference): Define
	as plain references for C++11 and later.
	(basic_string::basic_string()): Put constructor body outside
	preprocessor conditional groups.
	(basic_string::basic_string(basic_string&&)): Move _Alloc_hider
	instead of copying it.
	(basic_string::basic_string(const basic_string&, const _Alloc&)):
	Define.
	(basic_string::basic_string(basic_string&&, const _Alloc&)):
	Define.
	* include/bits/fs_path.h [!_GLIBCXX_USE_CXX11_ABI]: Remove special
	cases for old basic_string.
	* testsuite/21_strings/basic_string/cons/char/8.cc: Test
	allocator-extended constructors unconditionally. Add extra members to
	allocator type when using old string ABI.
	* testsuite/21_strings/basic_string/allocator/71964.cc: Enable test
	for old string ABI.
	* testsuite/21_strings/basic_string/cons/wchar_t/8.cc: Likewise.

From-SVN: r267584