Add a new function with which we can query if a Decl had been newly
created during the import process. This feature is a must if we want to
have a different static analysis strategy for such newly created
declarations.
This is a dependent patch that is needed for the new CTU implementation
discribed at
https://discourse.llvm.org/t/rfc-much-faster-cross-translation-unit-ctu-analysis-implementation/61728