Makes libc++ behavior consistent between C++03 and C++11.
Can use `decltype` in C++03 because `include/__config` defines a macro when
`decltype` is not available.
Reviewers: mclow.lists, EricWF, erik.pilkington, ldionne
Reviewed By: ldionne
Subscribers: dexonsmith, cfe-commits, howard.hinnant, ldionne, christof, jkorous, Quuxplusone
Differential Revision: https://reviews.llvm.org/D48753
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@349676 91177308-0d34-0410-b5e6-96231b3b80d8