C++1y: track object lifetime during constexpr evaluation, and don't allow
objects to be used once their lifetimes end. This completes the C++1y
constexpr extensions.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187025 91177308-0d34-0410-b5e6-96231b3b80d8