Fix checking for permitted results of constant expressions.

Fix checking for permitted results of constant expressions.

In the presence of mutable state, we need to check whether temporaries
involved in a constant expression have permissible values at the end of
the overall evaluation, rather than at the end of the evaluation of the
initializer of the temporary.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@373160 91177308-0d34-0410-b5e6-96231b3b80d8