[ASTImporter] Refactor ASTImporter to support custom downstream tests

Summary:
The purpose of this change is to do a small refactoring of code in
ASTImporterTest.cpp by moving it to ASTImporterFixtures.h in order to
support tests of downstream custom types and minimize the "living
downstream burden" of frequent integrations from community to a
downstream repo that implements custom AST import tests.

Reviewers: martong, a.sidorin, shafik

Reviewed By: martong

Subscribers: balazske, dkrupp, bjope, rnkovacs, teemperor, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83970