[clang] [CIR] Add NYI cases to builtin switch statement and move existing cases into functions (PR #168699)

[Hendrik Hübner via cfe-commits]

HendrikHuebner wrote:

> Thanks for doing this!
> 
> I appreciate the restructuring, but this is going to cause some problems for builtins that have a 1:1 mapping to library calls. Currently, we have a block of code following the switch statement to call the library function for cases that weren't previously handled.
> 
> ```
>   // If this is an alias for a lib function (e.g. __builtin_sin), emit
>   // the call using the normal call path, but using the unmangled
>   // version of the function name.
>   if (getContext().BuiltinInfo.isLibFunction(builtinID))
>     return emitLibraryCall(*this, fd, e,
>                            cgm.getBuiltinLibFunction(fd, builtinID));
> ```
> 
> If you put that inside of `errorBuiltinNYI` with a comment explaining that it's a temporary workaround, that will keep us from regressing on a bunch of tests that are currently passing in the llvm-test-suite.

I added `break;`s for the builtins that are handled by the piece of code you mentioned, e.g. `__builtin_printf` and the tests pass. What is  regressing here?

https://github.com/llvm/llvm-project/pull/168699