LWG3032: `ValueSwappable` requirement missing for `push_heap` and `make_heap`

[ldionne]

Link: https://wg21.link/LWG3032

[frederick-vs-ja]

This is somehow superfluous, since currently no mainstream implementation uses swap in heap algorithms. I guess it would be better to revert this LWG issue and remove other occurences of Cpp17ValueSwappable in the requirements for heap algorithms.