[ObjC] Allow declaring __strong pointer fields in structs in Objective-C
ARC mode.

Declaring __strong pointer fields in structs was not allowed in
Objective-C ARC until now because that would make the struct non-trivial
to default-initialize, copy/move, and destroy, which is not something C
was designed to do. This patch lifts that restriction.

Special functions for non-trivial C structs are synthesized that are
needed to default-initialize, copy/move, and destroy the structs and
manage the ownership of the objects the __strong pointer fields point
to. Non-trivial structs passed to functions are destructed in the callee
function.

rdar://problem/33599681

Differential Revision: https://reviews.llvm.org/D41228

llvm-svn: 326307
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index c4da87c..eb2c770 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -3929,7 +3929,9 @@
     : TagDecl(DK, TK, C, DC, IdLoc, Id, PrevDecl, StartLoc),
       HasFlexibleArrayMember(false), AnonymousStructOrUnion(false),
       HasObjectMember(false), HasVolatileMember(false),
-      LoadedFieldsFromExternalStorage(false) {
+      LoadedFieldsFromExternalStorage(false),
+      NonTrivialToPrimitiveDefaultInitialize(false),
+      NonTrivialToPrimitiveCopy(false), NonTrivialToPrimitiveDestroy(false) {
   assert(classof(static_cast<Decl*>(this)) && "Invalid Kind!");
 }