[clang] [CIR] Add cir.signbit to tablegen (PR #184091)

[Akimasa Watanuki via cfe-commits]

Men-cotton wrote:

Thank you for the contribution!

Could you also add a corresponding test for this change? You can base it on the existing test at: https://github.com/llvm/clangir/blob/HEAD/clang/test/CIR/CodeGen/builtin-signbit.c

https://github.com/llvm/llvm-project/pull/184091