Fix filesystem::path for iterators with const value_type

Fix filesystem::path for iterators with const value_type

	* include/experimental/bits/fs_path.h
	(path::_S_convert<_Iter>(_Iter, _Iter)): Remove cv-qualifiers from
	iterator's value_type.
	(path::_S_convert<_Iter>(_Iter __first, __null_terminated)): Likewise.
	Do not use operation not supported by input iterators.
	(path::__is_path_iter_src): Add partial specialization for const
	encoded character types.
	* testsuite/experimental/filesystem/path/construct/range.cc: Test
	construction from input iterators with const value types.

From-SVN: r241654