[ASTImporter] Convert ODR diagnostics inside ASTImporter implementation

Summary:
ASTStructuralEquivalence uses a flag to indicate whether ODR diagnostics
should be considered errors or warnings as module Sema is more strict than
ASTMerge. The implementation of ASTImporter should allso follow
along the same lines.

Reviewers: martong, a.sidorin, shafik, a_sidorin

Reviewed By: shafik, a_sidorin

Subscribers: rnkovacs, martong, dkrupp, Szelethus, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59761

Patch by Endre Fulop!

llvm-svn: 357402