[ASTImporter] Add test for C++ casts and fix broken const_cast importing.

Summary:
The ASTImporter does currently not handle const_casts. This patch adds the
missing const_cast importer code and the test case that discovered this.

Reviewers: a.sidorin, a_sidorin

Reviewed By: a_sidorin

Subscribers: a_sidorin, martong, cfe-commits

Differential Revision: https://reviews.llvm.org/D50932

llvm-svn: 340182