[ASTImporter] Add ms compatibility to tests which use the TestBase

Summary:
In order to avoid build failures on MS, we use -fms-compatibility too in
the tests which use the TestBase.  Moved the family of `testImport`
functions under a test fixture class, so we can use parameterized tests.
Refactored `testImport` and `testImportSequence`, because `for` loops over
the different compiler options is no longer needed, that is handeld by
the test framework via parameters from now on.

Reviewers: a.sidorin, r.stahl, xazax.hun

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D47367

llvm-svn: 335464