`<future>`: Consider supporting `promise<const int>` and `future<const int>`

[StephanTLavavej]

Followup to #2488 fixed by #2568. Nothing in the Standard appears to forbid promise<const int> and future<const int>, so they should work. It should be possible to slightly refine the new code added by #2568 to handle const qualifiers, but additional changes may be necessary, and test coverage is definitely necessary.

Noticed by Casey Carter (@CaseyCarter) in #2568 (comment) .

[fsb4000]

DevCom-10092031 : An user asks for the same.

[Pro]

See also corresponding question/issue report on StackOverflow:

https://stackoverflow.com/q/72894817/869402