This is a part of the RFC mentioned here - https://lists.llvm.org/pipermail/cfe-dev/2020-July/066203.html where we plan to move parts of Cling upstream.
Cling has the ability to spawn child interpreters (mainly for auto completions). We needed
to apply this patch on top of our fork of Clang, because otherwise when we try to
import a Decl into the Cling child interpreter using
Clang::ASTImporter, the assertion here - https://github.com/llvm/llvm-project/blob/65eb74e94b414fcde6bfa810d1c30c7fcb136b77/clang/include/clang/Basic/SourceLocation.h#L322 fails.