Patch for small addition to availability attribute.
This is to accept "NA" in place of vesion number for availability
attribute. Used on introduced=NA to mean unavailable
and deprecated=NA to mean nothing (not deprecated).
rdar://18804883


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221417 91177308-0d34-0410-b5e6-96231b3b80d8