[CodeView] Avoid NULL deref of Scope

Regression from D131400: cross-language LTO causes a crash in the
compiler on the NULL deref of Scope in `isa` call when Rust IR is
involved. Presumably, this might affect other languages too, and
even Rust itself without cross-language LTO when the Rust compiler
switched to LLVM 16.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D134616