[analyzer] Treat the rvalue of a forward-declared struct as Unknown.

This will never happen in the analyzed code code, but can happen for checkers
that over-eagerly dereference pointers without checking that it's safe.
UnknownVal is a harmless enough value to get back.

Fixes an issue added in r189590, caught by our internal buildbot.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189688 91177308-0d34-0410-b5e6-96231b3b80d8