ReExported symbols can point to a library that doesn't actually
contain the symbol, but just reexports wholesale from another
library.  Handle this case.

<rdar://problem/16977589>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@209270 91177308-0d34-0410-b5e6-96231b3b80d8