[ASTImporter][NFC] Add comprehensive tests for ODR violation handling strategies

Summary:
In this patch we provide additional and comprehensive tests for the ODR
handling strategies. This is the continuation of
https://reviews.llvm.org/D59692.

Reviewers: shafik, a_sidorin, balazske, a.sidorin

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66951

llvm-svn: 372564