[llvm] Fix debug info metadata for string types (PR #156379)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

Not sure this is right. My understanding is that extractMDNode is supposed to undo canonicalizeMetadataForValue(), which doesn't do anything for MDString.

Isn't the problem here just that you're missing one level of MDNode wrapper in your construction logic? After all, the value is not `!"Version 1.0"`, it's `!{!"Version 1.0"}`.

https://github.com/llvm/llvm-project/pull/156379