[Sema] Delay checking whether objc_designated_initializer is being applied to an init method

This fixes a regression that was caused by r335084, which reversed
the order that attributes are applied. objc_method_family can change
whether a method is an init method, so the order that these
attributes are applied matters. The commit fixes this by delaying the
init check until after all attributes have been applied.

rdar://47829358

Differential revision: https://reviews.llvm.org/D58152

llvm-svn: 353976