PR 91770 - emit a warning in a system header

[Jakub Jelinek]

On Thu, Oct 10, 2019 at 08:24:29AM -0400, Nathan Sidwell wrote:
> In addressing 91770, I need to emit a warning for a location inside a system
> header file[1].  The diagnostics machinery elides warnings when the location
> is in a system header :(
> 
> As I happen to have located the appropriate line_map in the process of
> determining the nested extern "C", I could clear the map's in-system-header
> flag around the warning emission.  But that's, well, urgh:(
> 
> Is there a neater way?

  bool save_warn_system_headers = global_dc->dc_warn_system_headers;
  global_dc->dc_warn_system_headers = 1;
...
  global_dc->dc_warn_system_headers = save_warn_system_headers;

?

I'm afraid warning_sentinel can't be used here, because that clears, rather
than sets it.

	Jakub