Define strnlen when it is not in libc

[Duncan P. N. Exon Smith via llvm-commits]

> On 2016-Nov-28, at 10:55, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> 
>> On 2016-Nov-24, at 11:10, Joerg Sonnenberger via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>> 
>> On Thu, Nov 24, 2016 at 02:06:55PM -0500, Chi-Hua Chen via llvm-commits wrote:
>>> Some older platforms, e.g. Mac 10.6.8, do not have strnlen. Define one
>>> in Support when necessary.
>> 
>> Personally, I'd prefer to just avoid it in first place. It is not really
>> much simpler than just using memchr.
> 
> `StringRef(...).find('\0')` seems cleaner.

But I'm fine with the original patch as well, since it avoids people introducing more uses of strnlen in the future... assuming we actually support building on 10.6?  (There weren't any buildbot failures...)

> 
>> 
>> Joerg
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