[ASTImporter] Add test helper Fixture

Add a helper test Fixture, so we can add tests which can check internal
attributes of AST nodes like getPreviousDecl(), isVirtual(), etc.
This enables us to check if a redeclaration chain is correctly built during
import, if the virtual flag is preserved during import, etc. We cannot check
such attributes with the existing testImport.
Also, this fixture makes it possible to import from several "From" contexts.

We also added several test cases here, some of them are disabled.
We plan to pass the disabled tests in other patches.

Patch by Gabor Marton!

Differential Revision: https://reviews.llvm.org/D43967

llvm-svn: 328906