[clang] [CIR] Support zero-result ops in clangir TableGen lowering (PR #202273)

[Sirui Mu via cfe-commits]

Lancern wrote:

> How should I reserve for the case of multiple results?

AFAIK all multi-value CIR operations are lowered to LLVM dialect with a non-"trivial" way. By "trivial" I mean that the lowering is done by simply replacing the original CIR operation with a corresponding LLVM operation, and this kind of trivial behavior is what the `llvmOp` mechanism tries to cover (at least for now).

So maybe we could just ignore the case of multiple results for now.

https://github.com/llvm/llvm-project/pull/202273