stage1: make C++ switch fallthrough an error

Make fallthrough an error when compiler supports it. This requires a new
macro that is defined with such compilers to be used as a statement, at
all fallthrough sites:

    switch (...) {
        case 0:
            ...
            ZIG_FALLTHROUGH;
        case 1:
            ...
            break;
        default:
            ...
            break;
    }

If we ever move to C++17 as minimal requirement, then the macro can be
replaced with `[[fallthrough]];` at statement sites.

src/codegen.cpp

@@ -3954,8 +3954,9 @@ static void render_async_var_decls(CodeGen *g, Scope *scope) {
                 if (var->did_the_decl_codegen) {
                     render_decl_var(g, var);
                 }
-                // fallthrough
             }
+            ZIG_FALLTHROUGH;
+
             case ScopeIdDecls:
             case ScopeIdBlock:
             case ScopeIdDefer: