[ASTImporter] Make ODR error handling configurable
ODR errors are not necessarily true errors during the import of ASTs.
ASTMerge and CrossTU should use the warning equivalent of every CTU error,
while Sema should emit errors as before.
Reviewers: martong, a_sidorin, shafik, a.sidorin
Reviewed By: a_sidorin
Subscribers: rnkovacs, dkrupp, Szelethus, jdoerfert, cfe-commits
Differential Revision: https://reviews.llvm.org/D58897
Patch by Endre Fulop!