[clang] [CIR][CodeGen] Implement __builtin_signbit (PR #188433)

[Yeongu Choe via cfe-commits]

YeonguChoe wrote:

@andykaylor 

Hello Andy

I fixed the comments. Please check.
One thing I am curious about is that what is target dependent values in CIR.
Is it okay to put values like `i64` or `0x7FF8000000000000` in the FileCheck?

https://github.com/llvm/llvm-project/pull/188433