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.

gcc/cp/call.cc

@@ -13539,6 +13539,13 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
     return;
   if (!TYPE_REF_P (TREE_TYPE (decl)))
     return;
+  /* Don't suppress the diagnostic just because the call comes from
+     a system header.  If the DECL is not in a system header, or if
+     -Wsystem-headers was provided, warn.  */
+  auto wsh
+    = make_temp_override (global_dc->dc_warn_system_headers,
+			  (!in_system_header_at (DECL_SOURCE_LOCATION (decl))
+			   || global_dc->dc_warn_system_headers));
   if (tree call = do_warn_dangling_reference (init))
     {
       auto_diagnostic_group d;

gcc/testsuite/g++.dg/warn/Wdangling-reference4.C

@@ -0,0 +1,14 @@
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wdangling-reference" }
+// Check that we warn here even without -Wsystem-headers.
+
+#include <optional>
+#include <string>
+
+auto f() -> std::optional<std::string>;
+
+void
+g ()
+{
+  for (char c : f().value()) { (void) c; } // { dg-warning "dangling reference" }
+}