Allow interfaces to operate on in-memory buffers with no source location info.

This patch is a part of the upstreaming efforts. Cling has the ability to spawn
child interpreters (mainly for auto completions). The child interpreter import
Decls using the ASTImporter which casuses the assertion here
https://github.com/llvm/llvm-project/blob/65eb74e94b414fcde6bfa810d1c30c7fcb136b77/clang/include/clang/Basic/SourceLocation.h#L322

The patch is co-developed with V. Vassilev.

Differential revision: https://reviews.llvm.org/D88780