Add replacement = "xxx" to AvailabilityAttr.

This commit adds a named argument to AvailabilityAttr, while r263652 adds an
optional string argument to __attribute__((deprecated)).

This was commited in r263687 and reverted in 263752 due to misaligned
access.

rdar://20588929


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263958 91177308-0d34-0410-b5e6-96231b3b80d8