[clang] [libc] [llvm] [clang] Make __builtin_expf constexpr. (PR #140841)

[Petr Hosek via cfe-commits]

petrhosek wrote:

I think the `errno` change should be extracted into a separate PR since that's related to libc internals and would make the actual change much smaller and easier to review.

https://github.com/llvm/llvm-project/pull/140841