[clang] [CIR] Convert null pointer constant types in target lowering (PR #228068)

[Arseniy Obolenskiy via cfe-commits]

https://github.com/aobolensk created https://github.com/llvm/llvm-project/pull/228068

The pass converted the cir.const result type but not the type inside its `#cir.ptr` attribute, so the verifier rejected address-space null constants

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