C++1y: Add a step limit to constexpr evaluation, to catch runaway loops.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181388 91177308-0d34-0410-b5e6-96231b3b80d8
Add test forgotten in r181388.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181389 91177308-0d34-0410-b5e6-96231b3b80d8