Define strnlen when it is not in libc

[Chi-Hua Chen via llvm-commits]

Duncan,

>> I have a question about the change in HeaderMap.cpp that introduced
>> the first though: why is '&& Data[Len - 1]' necessary?
>> https://github.com/llvm-mirror/clang/commit/839cd13b65d802f06276ff88d234419c38a44199#diff-860876c157b9f2ff75bce0d006cadc63
>
> Looks like it's unnecessary.  Do the tests pass without it?

Yes, 'make check' still pass without it.

- Chi-Hua