Allow ODR for ObjC/C in the sense that we won't keep more that one definition around (merge them). However, ensure the decl pass the structural compatibility check in C11 6.2.7/1, for that, reuse the structural equivalence checks used by the ASTImporter.
Few other considerations:
- Create error diagnostics for tag types mismatches and thread them into the structural equivalence checks.
- Note that by doing this we only support redefinition between types that are considered "compatible types" by C11.
This is mixed approach of the suggestions discussed in http://lists.llvm.org/pipermail/cfe-dev/2017-March/053257.html
Depends on D31777