Refactor path construction from null terminated iterator ranges

Refactor path construction from null terminated iterator ranges

Move duplicated code to new _S_string_from_iter function and fix
constraints to accept iterators with const value type.

	* include/bits/fs_path.h (path::__is_encoded_char): Change from class
	template to alias template.
	(path::__value_type_is_char): Use remove_const_t.
	(path:_S_string_from_iter): New helper function.
	(path::_S_convert(InputIter, __null_terminated))
	(path::_S_convert_loc(InputIter, __null_terminated, const locale&)):
	Use _S_string_from_iter.
	(path::string<_CharT, _Allocator>(const _Allocator&)): Allow sharing
	rep for COW strings.
	* include/experimental/bits/fs_path.h (path::__is_encoded_char):
	Change from class template to alias template.
	(path::__value_type_is_char): Use remove_const.
	(path:_S_string_from_iter): New helper function.
	(path::_S_convert(InputIter, __null_terminated))
	(path::_S_convert_loc(InputIter, __null_terminated, const locale&)):
	Use _S_string_from_iter.
	* testsuite/27_io/filesystem/path/append/source.cc: Test appending
	wide strings.
	* testsuite/27_io/filesystem/path/concat/strings.cc: Check for exact
	string equality, not path equivalence.
	* testsuite/27_io/filesystem/path/construct/format.cc: Check
	construction from std::string and std::wstring and input iterators.
	* testsuite/27_io/filesystem/path/construct/locale.cc: Check
	construction from iterators.
	* testsuite/experimental/filesystem/path/concat/strings.cc: Check for
	exact string equality, not path equivalence.
	* testsuite/experimental/filesystem/path/construct/locale.cc: Check
	construction from iterators.

From-SVN: r260628