[ObjC] Avoid the -Wunguarded-availability warnings for protocol

requirements in protocol/class/category declarations

The unguarded availability warnings in the protocol requirements of a protocol
/class/category declaration can be avoided. This matches the behaviour of
Swift's diagnostics. The warnings for deprecated/unavailable protocols are
preserved.

rdar://33156429

Differential Revision: https://reviews.llvm.org/D35061

llvm-svn: 307368