Dimitry Andric:  Add const to constexpr member functions in order to cope with new C++1y language rules.  This silences -Wconstexpr-not-const warnings.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185181 91177308-0d34-0410-b5e6-96231b3b80d8