This commit adds a named argument to AvailabilityAttr, while r263652 adds an
optional string argument to __attribute__((deprecated)). This enables the
compiler to provide Fix-Its for deprecated declarations.

rdar://20588929


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263687 91177308-0d34-0410-b5e6-96231b3b80d8