[clang] [CIR] Upstream minimal builtin function call support (PR #142981)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

There seems to be a LOT of code here that isn't in the tests?  The only thing tested is no argument types, and just an int return type, yet there is a ton of builtin code to do a lot more than that. 

I think this needs a lot more work to make sure we're properly testing this.

https://github.com/llvm/llvm-project/pull/142981