Unguarded availability diagnoser should use TraverseStmt instead of

Base::TraverseStmt when visiting the then/else branches of if statements

This ensures that the statement stack is correctly tracked and correct
multi-statement fixit is generated inside of an if (@available)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311088 91177308-0d34-0410-b5e6-96231b3b80d8