[libc++][istream] P3223R2: Making `std::istream::ignore` less surprising (#147007)
Implements https://wg21.link/P3223R2 as a DR as, as recommended in
https://github.com/cplusplus/papers/issues/1871#issuecomment-2993018698.
Resolves -1L ambiguity.
Closes #148178