Use Triple.isOSDarwin() instead of comparing against Triple::Darwin.

There are now separate Triple::MacOSX and Triple::IOS values for the OS
so comparing against Triple::Darwin will fail to match those.  Note that
I changed the expected output for the Driver/rewrite-objc.m test, which had
previously not been passing Darwin-specific options with the macosx triple.

llvm-svn: 141944