Failing Cling tests with multiple interpreters

[hahnjo]

Since the upgrade to LLVM 13, at least two tests are failing because of broken interactions with multiple interpreters:

• CodeUnloading/AtExit.C does not find the symbol gChild.
• MultipleInterpreters/MultipleInterpreters.C cannot find the symbol _ZN5cling7runtime6gClingE / cling::runtime::gCling during ChildInterp.echo() (the first usage to construct cling::Interpreter ChildInterp(*gCling, 1, argV); seems fine).

Based on this, one might theorize that the child interpreter doesn't have access to symbols declared in its parent (not sure how this worked before).