C backend: start handling anonymous decls

Start keeping track of dependencies on anon decls for dependency
ordering during flush()

Currently this causes use of undefined symbols because these
dependencies need to get rendered into the output.

src/Compilation.zig

@@ -3499,6 +3499,7 @@ fn processOneJob(comp: *Compilation, job: Job, prog_node: *std.Progress.Node) !v
                         .is_naked_fn = false,
                         .fwd_decl = fwd_decl.toManaged(gpa),
                         .ctypes = .{},
+                        .anon_decl_deps = .{},
                     };
                     defer {
                         dg.ctypes.deinit(gpa);