redecl chain for an imported declaration, make sure to check the IDNS of prior
imported decls.
Otherwise we can end up finding an invisible friend declaration and incorrectly
believing that it should be visible.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@321916 91177308-0d34-0410-b5e6-96231b3b80d8