[clang] Emit `diagnose_if` warnings from system headers

[EricWF]

Summary

In order for libc++ to meaningfully use diagnose_if warnings they need to be emitted from system headers by default. This patch changes the diagnose_if warning diagnostic to be shown in system headers.

[aaron.ballman]

LGTM!

include/clang/Basic/DiagnosticSemaKinds.td

 def note_ovl_candidate_substitution_failure : Note<
     "candidate template ignored: substitution failure%0%1">;
 def note_ovl_candidate_disabled_by_enable_if : Note<
     "candidate template ignored: disabled by %0%1">;
 def note_ovl_candidate_has_pass_object_size_params: Note<
     "candidate address cannot be taken because parameter %0 has "
     "pass_object_size attribute">;
 def err_diagnose_if_succeeded : Error<"%0">;
-def warn_diagnose_if_succeeded : Warning<"%0">, InGroup<UserDefinedWarnings>;
+def warn_diagnose_if_succeeded : Warning<"%0">, InGroup<UserDefinedWarnings>,
+    ShowInSystemHeader;
 def note_ovl_candidate_disabled_by_function_cond_attr : Note<
     "candidate disabled: %0">;
 def note_ovl_candidate_disabled_by_extension : Note<
     "candidate disabled due to OpenCL extension">;
 def err_addrof_function_disabled_by_enable_if_attr : Error<
     "cannot take address of function %0 becuase it has one or more "
     "non-tautological enable_if conditions">;
 def note_addrof_ovl_candidate_disabled_by_enable_if_attr : Note<

test/Sema/Inputs/diagnose-if-warn-system-header.h

+#pragma GCC system_header
+
+inline int system_header_func(int x)
+  __attribute__((diagnose_if(x == x, "system header warning", "warning"))) // expected-note {{from 'diagnose_if' attribute}}
+{
+  return 0;
+}
+
+void test_system_header() {
+  system_header_func(0); // expected-warning {{system header warning}}
+}

test/Sema/diagnose_if.c

 void runNoMsg() {
   noMsg(1); // expected-warning{{<no message provided>}}
 }
 
 void alwaysWarnWithArg(int a) _diagnose_if(1 || a, "alwaysWarn", "warning"); // expected-note{{from 'diagnose_if'}}
 void runAlwaysWarnWithArg(int a) {
   alwaysWarnWithArg(a); // expected-warning{{alwaysWarn}}
 }
+
+// Test that diagnose_if warnings generated in system headers are not ignored.
+#include "Inputs/diagnose-if-warn-system-header.h"