[clang] [Clang] Adjust `exit()` builtin impl (PR #101689)

[Dmitry Chestnykh via cfe-commits]

chestnykh wrote:

> In #74803, the question of "why" came up and was never answered.

Okay, i can remove `let AddBuiltinPrefixedAlias = 1;`, but IMO that the rest of changes makes sense. What do you think? 

https://github.com/llvm/llvm-project/pull/101689