Improve diagnostic for ignored GNU 'used' attribute

[aaron.ballman]

LGTM, thank you, that's a much better diagnostic than the original!

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def warn_dllimport_dropped_from_inline_function : Warning<
   "%q0 redeclared inline; %1 attribute ignored">,
   InGroup<IgnoredAttributes>;
 def warn_attribute_ignored : Warning<"%0 attribute ignored">,
   InGroup<IgnoredAttributes>;
 def warn_nothrow_attribute_ignored : Warning<"'nothrow' attribute conflicts with"
   " exception specification; attribute ignored">,
   InGroup<IgnoredAttributes>;
+def warn_attribute_ignored_on_non_definition :
+  Warning<"%0 attribute ignored on a non-definition declaration">,
+  InGroup<IgnoredAttributes>;
 def warn_attribute_ignored_on_inline :
   Warning<"%0 attribute ignored on inline function">,
   InGroup<IgnoredAttributes>;
 def warn_nocf_check_attribute_ignored :
   Warning<"'nocf_check' attribute ignored; use -fcf-protection to enable the attribute">,
   InGroup<IgnoredAttributes>;
 def warn_attribute_after_definition_ignored : Warning<
   "attribute %0 after definition is ignored">,

clang/lib/Sema/SemaDecl.cpp

if (F && getDLLAttr(F)) {
       Diag(VD->getLocation(), diag::err_attribute_dll_thread_local) << VD
                                                                     << DLLAttr;
       VD->setInvalidDecl();
     }
   }
 
   if (UsedAttr *Attr = VD->getAttr<UsedAttr>()) {
     if (!Attr->isInherited() && !VD->isThisDeclarationADefinition()) {
-      Diag(Attr->getLocation(), diag::warn_attribute_ignored) << Attr;
+      Diag(Attr->getLocation(), diag::warn_attribute_ignored_on_non_definition)
+          << Attr;
       VD->dropAttr<UsedAttr>();
     }
   }
 
   const DeclContext *DC = VD->getDeclContext();
   // If there's a #pragma GCC visibility in scope, and this isn't a class
   // member, set the visibility of this variable.
   if (DC->getRedeclContext()->isFileContext() && VD->isExternallyVisible())

clang/test/Sema/attr-used.c

 // RUN: %clang_cc1 -verify -fsyntax-only -Wno-private-extern %s
 
-extern int l0 __attribute__((used)); // expected-warning {{'used' attribute ignored}}
-__private_extern__ int l1 __attribute__((used)); // expected-warning {{'used' attribute ignored}}
+extern int l0 __attribute__((used)); // expected-warning {{'used' attribute ignored on a non-definition declaration}}
+__private_extern__ int l1 __attribute__((used)); // expected-warning {{'used' attribute ignored on a non-definition declaration}}
 
 struct __attribute__((used)) s { // expected-warning {{'used' attribute only applies to variables with non-local storage, functions, and Objective-C methods}}
   int x;
 };
 
 int a __attribute__((used));
 
 static void __attribute__((used)) f0(void) {
 }
 
 void f1() {
   static int a __attribute__((used));
   int b __attribute__((used)); // expected-warning {{'used' attribute only applies to variables with non-local storage, functions, and Objective-C methods}}
 }
 
 static void __attribute__((used)) f0(void);

clang/test/SemaCXX/attr-used.cpp

 // RUN: %clang_cc1 -fsyntax-only -verify %s
 
-extern char test1[] __attribute__((used)); // expected-warning {{'used' attribute ignored}}
-extern const char test2[] __attribute__((used)); // expected-warning {{'used' attribute ignored}}
+extern char test1[] __attribute__((used)); // expected-warning {{'used' attribute ignored on a non-definition declaration}}
+extern const char test2[] __attribute__((used)); // expected-warning {{'used' attribute ignored on a non-definition declaration}}
 extern const char test3[] __attribute__((used)) = "";