Add noexcept to filesystem::path query functions

Add noexcept to filesystem::path query functions

In the standard these member functions are specified in terms of the
potentially-throwing path decompositions functions, but we implement
them without constructing any new paths or doing anything else that can
throw.

	PR libstdc++/71044
	* include/bits/fs_path.h (path::has_root_name)
	(path::has_root_directory, path::has_root_path)
	(path::has_relative_path, path::has_parent_path)
	(path::has_filename, path::has_stem, path::has_extension)
	(path::is_absolute, path::is_relative, path::_M_find_extension): Add
	noexcept.
	* src/c++17/fs_path.cc (path::has_root_name)
	(path::has_root_directory, path::has_root_path)
	(path::has_relative_path, path::has_parent_path)
	(path::has_filename, path::_M_find_extension): Add noexcept.

From-SVN: r268713