[sema] Allow to opt-out of overriding the super class's designated...

[sema] Allow to opt-out of overriding the super class's designated initializers by marking the initializer as unavailable in the subclass.

rdar://20281322

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243676 91177308-0d34-0410-b5e6-96231b3b80d8