[CTU] Add more lit tests and better error handling

Summary:
Adding some more CTU list tests. E.g. to check if a construct is unsupported.
We also slightly modify the handling of the return value of the `Import`
function from ASTImporter.

Reviewers: xazax.hun, balazske, a_sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Differential Revision: https://reviews.llvm.org/D55131

llvm-svn: 348605