[ObjC] Allow declaring __weak pointer fields in C structs in ARC.

This patch uses the infrastructure added in r326307 for enabling
non-trivial fields to be declared in C structs to allow __weak fields in
C structs in ARC.

This recommits r327206, which was reverted because it caused
module-enabled builders to fail. I discovered that the
CXXRecordDecl::CanPassInRegisters flag wasn't being set correctly in
some cases after I moved it to RecordDecl.

Thanks to Eric Liu for helping me investigate the bug.

rdar://problem/33599681

https://reviews.llvm.org/D44095

llvm-svn: 327870
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index e141870..41b47ea 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -3553,6 +3553,12 @@
   bool NonTrivialToPrimitiveCopy : 1;
   bool NonTrivialToPrimitiveDestroy : 1;
 
+  /// True if this class can be passed in a non-address-preserving fashion
+  /// (such as in registers).
+  /// This does not imply anything about how the ABI in use will actually
+  /// pass an object of this class.
+  bool CanPassInRegisters : 1;
+
 protected:
   RecordDecl(Kind DK, TagKind TK, const ASTContext &C, DeclContext *DC,
              SourceLocation StartLoc, SourceLocation IdLoc,
@@ -3636,6 +3642,18 @@
     NonTrivialToPrimitiveDestroy = V;
   }
 
+  /// Determine whether this class can be passed in registers. In C++ mode,
+  /// it must have at least one trivial, non-deleted copy or move constructor.
+  /// FIXME: This should be set as part of completeDefinition.
+  bool canPassInRegisters() const {
+    return CanPassInRegisters;
+  }
+
+  /// Set that we can pass this RecordDecl in registers.
+  void setCanPassInRegisters(bool CanPass) {
+    CanPassInRegisters = CanPass;
+  }
+
   /// \brief Determines whether this declaration represents the
   /// injected class name.
   ///