Fix crash-on-invalid if decltype(auto) is used as a deduced return type in
C++11 mode. Continue to disallow return type deduction in C++11 for now.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201438 91177308-0d34-0410-b5e6-96231b3b80d8