Darwin: -Wformat-diag fix (PR93641)

The length used for the comparison for 'CFStringRef' was only comparing
for 'CFString', potentially allowing mismatched identifiers.

2020-02-10  Iain Sandoe  <iain@sandoe.co.uk>

	PR other/93641
	* config/darwin-c.c (darwin_cfstring_ref_p): Fix up last
	argument of strncmp.

gcc/ChangeLog

+2020-02-10  Iain Sandoe  <iain@sandoe.co.uk>
+
+	PR other/93641
+	* config/darwin-c.c (darwin_cfstring_ref_p): Fix up last
+	argument of strncmp.
+
 2020-02-10  Hans-Peter Nilsson  <hp@axis.com>
 
 	Try to generate zero-based comparisons.

gcc/config/darwin-c.c

@@ -809,7 +809,8 @@ darwin_cfstring_ref_p (const_tree strp)
     tn = DECL_NAME (tn);
   return (tn 
 	  && IDENTIFIER_POINTER (tn)
-	  && !strncmp (IDENTIFIER_POINTER (tn), "CFStringRef", 8));
+	  && !strncmp (IDENTIFIER_POINTER (tn), "CFStringRef",
+		       strlen ("CFStringRef")));
 }
 
 /* At present the behavior of this is undefined and it does nothing.  */