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

[via cfe-commits]

lntue 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.

Yes, I do plan to split the commits into their own PRs.  But probably I should go ahead and do the libc_errno refactoring first to reduce the noise for this proof-of-concept PR.

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