[ASTImporter] Add test RedeclChainShouldBeCorrectAmongstNamespaces

Summary:
We add a new test to show that redecl chains are not handled properly
amongst namespaces. We cannot pass this test now, so this is disabled.
Subsequent patches will make this test pass.

Reviewers: a_sidorin, shafik, a.sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57901

llvm-svn: 353684