[ASTImporter] Separate unittest files

Summary:
Move generic redecl chain tests and visibility tests into their own
separate test files.

Reviewers: a_sidorin, a.sidorin, shafik

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61786

llvm-svn: 360572