platform implies default visibility. To achieve these, refactor our
lookup of explicit visibility so that we search for both an explicit
VisibilityAttr and an appropriate AvailabilityAttr, favoring the
VisibilityAttr if it is present.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@128336 91177308-0d34-0410-b5e6-96231b3b80d8
Douglas Gregor