[clang] [CIR] Allow only CIR types in function signatures (PR #215922)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

After updating this to fix the build problem (I needed to account for the new `member_kind` argument for structs and unions), I found that a number of CIR tests were failing with this patch applied because we were creating a runtime function in LoweringPrepare with a pointer to a builtin MLIR type as part of its signature. That is fixed by https://github.com/llvm/llvm-project/pull/217790. I'll update this PR after that one lands.

https://github.com/llvm/llvm-project/pull/215922