[Modules][Objective-C] Use complete decl from module when diagnosing missing import

[bruno]

Summary

Otherwise the definition (first found) for ObjCInterfaceDecl's might
precede the module one, which will eventually lead to crash, since
diagnoseMissingImport needs one coming from a module.

This behavior changed after Richard's r342018, which started to look
into the definition of ObjCInterfaceDecls.

rdar://problem/49237144

[bruno]

Ping!

[arphaman]

LGTM

cfe/trunk/lib/Sema/SemaLookup.cpp

 /// the given declaration.
 static NamedDecl *getDefinitionToImport(NamedDecl *D) {
   if (VarDecl *VD = dyn_cast<VarDecl>(D))
     return VD->getDefinition();
   if (FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
     return FD->getDefinition();
   if (TagDecl *TD = dyn_cast<TagDecl>(D))
     return TD->getDefinition();
+  // The first definition for this ObjCInterfaceDecl might be in the TU
+  // and not associated with any module. Use the one we know to be complete
+  // and have just seen in a module.
   if (ObjCInterfaceDecl *ID = dyn_cast<ObjCInterfaceDecl>(D))
-    return ID->getDefinition();
+    return ID;
   if (ObjCProtocolDecl *PD = dyn_cast<ObjCProtocolDecl>(D))
     return PD->getDefinition();
   if (TemplateDecl *TD = dyn_cast<TemplateDecl>(D))
     if (NamedDecl *TTD = TD->getTemplatedDecl())
       return getDefinitionToImport(TTD);
   return nullptr;
 }
 

cfe/trunk/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Headers/Bar.h

+// interface-diagnose-missing-import/Foo.framework/Headers/Bar.h

cfe/trunk/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Headers/Foo.h

+#import <Foo/RandoPriv.h>
+#import <Foo/Bar.h>

cfe/trunk/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Modules/module.modulemap

+// interface-diagnose-missing-import/Foo.framework/Modules/module.modulemap
+framework module Foo {
+  umbrella header "Foo.h"
+  export *
+  module * { export * }
+}

cfe/trunk/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/PrivateHeaders/RandoPriv.h

+@interface NSObject
+@end
+@interface Buggy : NSObject
+@end

cfe/trunk/test/Modules/interface-diagnose-missing-import.m

+// RUN: rm -rf %t
+// RUN: %clang_cc1 %s -fsyntax-only -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -F%S/Inputs/interface-diagnose-missing-import -verify
+@interface Buggy
+@end
+
+@import Foo.Bar;
+
+@interface Buggy (MyExt) // expected-error {{definition of 'Buggy' must be imported from module 'Foo' before it is required}}
+@end
+
+// expected-note@Foo/RandoPriv.h:3{{previous definition is here}}