PR libstdc++/84159 fix appending strings to paths

PR libstdc++/84159 fix appending strings to paths

The path::operator/=(const Source&) and path::append overloads were
still following the semantics of the Filesystem TS not C++17. Only
the path::operator/=(const path&) overload was correct.

This change adds more tests for path::operator/=(const path&) and adds
new tests to verify that the other append operations have equivalent
behaviour.

	PR libstdc++/84159
	* include/bits/fs_path.h (path::operator/=, path::append): Construct
	temporary path before calling _M_append.
	(path::_M_append): Change parameter to path and implement C++17
	semantics.
	* testsuite/27_io/filesystem/path/append/path.cc: Add helper function
	and more examples from the standard.
	* testsuite/27_io/filesystem/path/append/source.cc: New.
	* testsuite/27_io/filesystem/path/decompose/filename.cc: Add comment.
	* testsuite/27_io/filesystem/path/nonmember/append.cc: New.

From-SVN: r260255