[clang] C89 doesn't have `math.h` functions (PR #129979)

[Vinay Deshmukh via cfe-commits]

vinay-deshmukh wrote:


I'm a bit confused about the whole what is not UB/IFNDR in C89, but is in C99. Based on my current understanding the simplest way to resolve the issue seems to what I've done as of: 
7572c21

Is not define builtins for symbols available in C89?

let me know if that looks good.

https://github.com/llvm/llvm-project/pull/129979