c++: -Wdangling-reference and system headers

I got this testcase:

  auto f() -> std::optional<std::string>;
  for (char c : f().value()) { }

which has a dangling reference: std::optional<T>::value returns
a reference to the contained value, but here it's the f() temporary.
We warn, which is great, but only with -Wsystem-headers, because
the function comes from a system header and warning_enabled_at used
in do_warn_dangling_reference checks diagnostic_report_warnings_p,
which in this case returned false so we didn't warn.

Fixed as below.  I could also override dc_warn_system_headers so that
the warning is enabled in system headers always.  With that, I found one
issue in libstdc++:

libstdc++-v3/include/bits/fs_path.h:1265:15: warning: possibly dangling reference to a temporary [-Wdangling-reference]
 1265 |         auto& __last = *--end();
      |               ^~~~~~

which looks like a true positive as well.

gcc/cp/ChangeLog:

	* call.cc (maybe_warn_dangling_reference): Enable the warning in
	system headers if the decl isn't in a system header.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wdangling-reference4.C: New test.