[ASTImporter] Make ODR error handling configurable

Summary:
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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58897

Patch by Endre Fulop!

llvm-svn: 357394