Only merge down a variable type if the previous declaration was
visible.  There's a lot of potential badness in how we're modelling
these things, but getting this much correct is reasonably easy.

rdar://13535367

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178488 91177308-0d34-0410-b5e6-96231b3b80d8