[Sema] -Wformat-pedantic only for NSInteger/NSUInteger %tu/%td on Darwin

The '%tu'/'%td' as formatting specifiers have been used to print out the
NSInteger/NSUInteger values for a long time. Typically their ABI matches, but that's
not the case on watchOS. The ABI difference boils down to the following:

- Regular 32-bit darwin targets (like armv7) use 'ptrdiff_t' of type 'int',
  which matches 'NSInteger'.
- WatchOS arm target (armv7k) uses 'ptrdiff_t' of type 'long', which doesn't
  match 'NSInteger' of type 'int'.

Because of this ABI difference these specifiers trigger -Wformat warnings only
for watchOS builds, which is really inconvenient for cross-platform code.

This patch avoids this -Wformat warning for '%tu'/'%td' and NS[U]Integer only,
and instead uses the new -Wformat-pedantic warning that JF introduced in
https://reviews.llvm.org/D47290. This is acceptable because Darwin guarantees that,
despite the watchOS ABI differences, sizeof(ptrdiff_t) == sizeof(NS[U]Integer),
and alignof(ptrdiff_t) == alignof(NS[U]Integer) so the warning is therefore noisy
for pedantic reasons.

I'll update public documentation to ensure that this behaviour is properly
communicated.

rdar://41739204

Differential Revision: https://reviews.llvm.org/D48852

llvm-svn: 336396