`<expected>`: Add deleted function overloads to `expected`

[frederick-vs-ja]

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Currently,

• libc++'s expected has all deleted overloads specified in the standard, including the inconsistent deleted overload of move assignment operator.
• libstdc++'s expected seems to have all but the inconsistent one (the defaulted overloads are sometimes deleted).
• MSVC STL's expected lacks deleted overloads (including those of copy constructors). My recent PR #4271 doesn't change this.

Given that the standard generally doesn't make move functions conditionally deleted, I tried to submit an LWG issue to constrain the move assignment operator of expected<cv void, E>. Edit: this is now LWG-4025.

It seems that the existence of deleted overloads can affect overload resolution in some corner cases, e.g. when someone uses a class derived from expected with using expected::operator=;. On the other hand, we can properly constrain copy functions since C++20, so it's doubtful whether expected should have deleted overloads.

[StephanTLavavej]

We talked about this at the weekly maintainer meeting and we believe that we should Do What The Standard Says(TM) here, except when the Standard depicts deleted move operations (@CaseyCarter strongly believes that the Standard is bogus whenever it depicts that).