frontend: skip astgen for builtin.zig

since it's already done ahead of time and always unchanging

src/Module.zig

@@ -2637,6 +2637,8 @@ comptime {
 }
 
 pub fn astGenFile(mod: *Module, file: *File) !void {
+    assert(!file.mod.isBuiltin());
+
     const tracy = trace(@src());
     defer tracy.end();