[Availability] Improve availability to consider functions run at load time

Summary:
There are some functions/methods that run when the application launches
or the library loads. Those functions will run reguardless the OS
version as long as it satifies the minimum deployment target. Annotate
them with availability attributes doesn't really make sense because they
are essentially available on all targets since minimum deployment
target.

rdar://problem/36093384

Reviewers: arphaman, erik.pilkington

Reviewed By: erik.pilkington

Subscribers: erik.pilkington, cfe-commits

Differential Revision: https://reviews.llvm.org/D45699

llvm-svn: 330166