Fix spurious warnings about unused private field

[ogoffart]

Summary

If the address of a field is taken as a pointer to member, we should
not warn that the field is not used.

Normaly, yse of fields are done from MemberExpr, but in case of pointer to
member, it is in a DeclRefExpr

[djasper]

Looks good.

lib/Sema/SemaExpr.cpp

       !Diags.isIgnored(diag::warn_arc_repeated_use_of_weak, E->getLocStart()))
       recordUseOfEvaluatedWeak(E);
 
-  // Just in case we're building an illegal pointer-to-member.
-  FieldDecl *FD = dyn_cast<FieldDecl>(D);
-  if (FD && FD->isBitField())
-    E->setObjectKind(OK_BitField);
+  if (FieldDecl *FD = dyn_cast<FieldDecl>(D)) {
+    UnusedPrivateFields.remove(FD);
+    // Just in case we're building an illegal pointer-to-member.
+    if (FD->isBitField())
+      E->setObjectKind(OK_BitField);
+  }
 
   return E;
 }

test/SemaCXX/warn-unused-private-field.cpp

     int *use = &by_reference_;
     int test[2];
     test[as_array_index_] = 42;
+    int EverythingUsed::*ptr = &EverythingUsed::by_pointer_to_member_;
   }
 
   template<class T>

   int by_template_function_;
   int as_array_index_;
   int by_initializer_;
+  int by_pointer_to_member_;
 };
 
 class HasFeatureTest {