The resolution for LWG 2936 defines the comparison more precisely, which
this patch implements. The patch also defines comparisons with strings
to work without constructing a temporary path object (so avoids any
memory allocations).

	* include/bits/fs_path.h (path::compare(const string_type&))
	(path::compare(const value_type*)): Add noexcept and construct a
	string view to compare to instead of a path.
	(path::compare(basic_string_view<value_type>)): Add noexcept. Remove
	inline definition.
	* src/filesystem/std-path.cc (path::_Parser): Track last type read
	from input.
	(path::_Parser::next()): Return a final empty component when the
	input ends in a non-root directory separator.
	(path::_M_append(basic_string_view<value_type>)): Remove special cases
	for trailing non-root directory separator.
	(path::_M_concat(basic_string_view<value_type>)): Likewise.
	(path::compare(const path&)): Implement LWG 2936.
	(path::compare(basic_string_view<value_type>)): Define in terms of
	components returned by parser, consistent with LWG 2936.
	* testsuite/27_io/filesystem/path/compare/lwg2936.cc: New.
	* testsuite/27_io/filesystem/path/compare/path.cc: Test more cases.
	* testsuite/27_io/filesystem/path/compare/strings.cc: Likewise.

From-SVN: r267235