More -Werror,-Wunused-private-field

...detected with a modified trunk Clang with

> Index: lib/Sema/SemaDeclCXX.cpp
> ===================================================================
> --- lib/Sema/SemaDeclCXX.cpp	(revision 219190)
> +++ lib/Sema/SemaDeclCXX.cpp	(working copy)
> @@ -1917,9 +1917,10 @@
>    const Type *T = FD.getType()->getBaseElementTypeUnsafe();
>    // FIXME: Destruction of ObjC lifetime types has side-effects.
>    if (const CXXRecordDecl *RD = T->getAsCXXRecordDecl())
> -    return !RD->isCompleteDefinition() ||
> -           !RD->hasTrivialDefaultConstructor() ||
> -           !RD->hasTrivialDestructor();
> +    return !RD->hasAttr<WarnUnusedAttr>() &&
> +           (!RD->isCompleteDefinition() ||
> +            !RD->hasTrivialDefaultConstructor() ||
> +            !RD->hasTrivialDestructor());
>    return false;
>  }
>
> @@ -3517,9 +3518,11 @@
>    bool addFieldInitializer(CXXCtorInitializer *Init) {
>      AllToInit.push_back(Init);
>
> +#if 0
>      // Check whether this initializer makes the field "used".
>      if (Init->getInit()->HasSideEffects(S.Context))
>        S.UnusedPrivateFields.remove(Init->getAnyMember());
> +#endif
>
>      return false;
>    }

to warn about members of SAL_WARN_UNUSED-annotated class types, and warn about
initializations with side effects (cf.
<http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-October/039602.html>
"-Wunused-private-field distracted by side effects").

Change-Id: I3f3181c4eb8180ca28e1fa3dffc9dbe1002c6628