[ASTImporter] Propagate errors during import of overridden methods.

Summary:
If importing overridden methods fails for a method it can be seen
incorrectly as non-virtual. To avoid this inconsistency the method
is marked with import error to avoid later use of it.

Reviewers: martong, a.sidorin, shafik, a_sidorin

Reviewed By: martong, shafik

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66933

llvm-svn: 370457