[libc++] reference type of the input range (and _It) isn't necessarily const _ValueT&

[fsb4000]

While I did https://reviews.llvm.org/D142864 I add some static_casts.
Casey Carter explained why they are needed (the explanation is about Microsoft STL but it is the same for libc++):

We get here from one of two places: the initializer_list overload of operator() defined on 10084, or the range overload defined on 10096. If it's the former, we know that the value type of the range _Vty is copyable and the reference type is const _Vty& so *_Result.first implicitly converts to _Vty - we don't need the static_cast.

In the latter case, indirectly_copyable_storable<_It, _Vty*> (https://eel.is/c++draft/alg.req.ind.copy#2) also requires that _Vty is copyable, but the reference type of the input range (and _It) isn't necessarily const _Vty& so we don't have the same guarantee of implicit convertibility. In fact there's no requirement on that algorithm that lets us implicitly convert the result of an iterator dereference to _Vty. There is a requirement that constructible_from<_Vty, iter_reference_t<_It>> holds, however, so we can perform the conversion explicitly with static_cast.

An example of affected code:

llvm-project/libcxx/include/__algorithm/ranges_minmax.h

Line 80 in 4afb1ee

return {*__result.first, *__result.second};

philnik wrote that we should check all algorithms for the issue.