enable use_llvm if clang c_frontend is required

[xxxbxxx]

fix zig cc -fstack-protector

was failing since 53f74d6, with error: unable to create compilation: StackProtectorUnsupportedByTarget

also fixes zig build-exe that no longer enabled stack-protector by default when compiling c files.

closes #18009
closes #18114

[andrewrk]

Thanks for looking into this.

Let's discuss here but then I'll need to incorporate this change into #18160 which directly conflicts.

I don't think this is the correct fix. use_llvm only indicates whether the LLVM backend will be used for compiling .zig code. It should have no effect on the C frontend.

Eyeballing the diff from 53f74d6, it looks to me like the regression came from target_util.supportsStackProtector correctly noticing which zig backend is being used to generate code, while the logic calling that function incorrectly assumes that .zig code is being compiled, when in reality it might be only C code being compiled.

So I think the correct fix is also to introduce another "supports stack protector" function to target_util for C compilation, and this one will take the c_frontend as a parameter. Then the logic will look something like this:

if (have_zcu and !target_util.zigSupportsStackProtector(target, zig_backend)) {
    // then disable it and error if explicitly requested
}
if (have_any_c_files and !target_util.cSupportsStackProtector(target, c_frontend, zig_backend)) {
    // then disable it and error if explicitly requested
}

have_zcu is "have a zig compilation unit" i.e. there is any .zig code being compiled. src/Module.zig will be renamed to Zcu soon.

[xxxbxxx]

Thanks for looking into this.

Let's discuss here but then I'll need to incorporate this change into #18160 which directly conflicts.

ok! I'll update this once #18160 has landed.

I don't think this is the correct fix. use_llvm only indicates whether the LLVM backend will be used for compiling .zig code. It should have no effect on the C frontend.

yeah I wasn't so sure about the meaning of use_llvm. thanks for clarifying.