[Sema] -Wformat-pedantic only for NSInteger/NSUInteger %zu/%zi on Darwin

Summary:
Pick D42933 back up, and make NSInteger/NSUInteger with %zu/%zi specifiers on Darwin warn only in pedantic mode. The default -Wformat recently started warning for the following code because of the added support for analysis for the '%zi' specifier.

     NSInteger i = NSIntegerMax;
     NSLog(@"max NSInteger = %zi", i);

The problem is that on armv7 %zi is 'long', and NSInteger is typedefed to 'int' in Foundation. We should avoid this warning as it's inconvenient to our users: it's target specific (happens only on armv7 and not arm64), and breaks their existing code. We should also silence the warning for the '%zu' specifier to ensure consistency. This is acceptable because Darwin guarantees that, despite the unfortunate choice of typedef, sizeof(size_t) == sizeof(NS[U]Integer), the warning is therefore noisy for pedantic reasons. Once this is in I'll update public documentation.

Related discussion on cfe-dev:
http://lists.llvm.org/pipermail/cfe-dev/2018-May/058050.html

<rdar://36874921&40501559>

Reviewers: ahatanak, vsapsai, alexshap, aaron.ballman, javed.absar, jfb, rjmccall

Subscribers: kristof.beyls, aheejin, cfe-commits

Differential Revision: https://reviews.llvm.org/D47290

llvm-svn: 335393