Tighten types a bit. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171895 91177308-0d34-0410-b5e6-96231b3b80d8
Don't crash when trying to apply the availability attribute to a block.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171899 91177308-0d34-0410-b5e6-96231b3b80d8