[llvm] [DebugInfo] Fix compact DWARF expression failure handling (PR #213391)

[Scott Linder via llvm-commits]

https://github.com/slinder1 commented:

This isn't exactly what I had in mind, but it is another stable place to be (assuming we document the fact that the `GetReg` callback should be "pure").

https://github.com/llvm/llvm-project/pull/213391