[ASTImporter] Eliminated some unittest warnings.

Summary:
When running the ASTTests test, warnings produced by the compiler can be
distracting when looking for test errors. A part of the warnings is removed
by setting extra compiler options.

Reviewers: a.sidorin

Reviewed By: a.sidorin

Subscribers: a_sidorin, martong, cfe-commits

Differential Revision: https://reviews.llvm.org/D47459

Patch by Balazs Keri!

llvm-svn: 335959