[exec.util.cmplsig.trans] Replace verbose add_lvalue_reference_t with plain &

[frederick-vs-ja]

In the example in [exec.util.cmplsig.trans], the fallback mechanism of add_lvalue_reference_t doesn't make sense, because when set_value_t(Args&...) is an invalid type, set_value_t(add_lvalue_reference_t<Args>...) can't be valid. Although use of add_lvalue_reference_t possibly triggers more accepts-invalid compiler bugs.

Note that other uses of add_lvalue_reference_t are meaningful:

• For common_with and common_referene, uses of add_lvalue_reference_t are necessary to make common_with<void, void> satisfied.
• For unique_ptr, it is possible to form a unique_ptr<void, D> whose operator* is well-formed and returns void, as D::pointer can also have an operator* returning void (demo).