Modern gcc is happy to constant evaluate __builtin_strlen in various cases
where we didn't. Extend our constant evaluation for __builtin_strlen to handle
any constant array of chars, not just string literals, to match.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194762 91177308-0d34-0410-b5e6-96231b3b80d8