[clang][analyzer] Test cases for CTU import failures
Add tests for all kinds of CTU import failures expressed in the
`cross_tu::index_error_code` enumeration. Many of the test cases have no
diagnostics, because the respective errors are silently ignored in
`CrossTranslationUnitContext::emitCrossTUDiagnostics`.
I expect to improve this part, and these tests should demonstrate how
the improvements manifest to the user.
--
CPP-7804
