[ASTImporter] Add unit tests for structural equivalence

Summary:
This patch add new tests for structural equivalence. For that a new
common header is created which holds the test related language specific
types and functions.

Reviewers: a.sidorin, xazax.hun, szepet

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D46867

llvm-svn: 333166