[llvm] [AsmParser] Add support for reading incomplete IR (part 1) (PR #78421)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle approved this pull request.

Yeah, I can see how this can be convenient sometimes. LGTM.

(I wonder if this can be made more discoverable. Perhaps by outputting a hint about this option when undefined metadata is encountered.)

https://github.com/llvm/llvm-project/pull/78421