Fix #2928: Add special mode for computing the type of a LHS
Fix #2928: Add special mode for computing the type of a LHS

To fix #2928, we need to compute the member type of an assigment's
left-hand side using negative variance.

dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,som-snytt/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,dotty-staging/dotty,lampepfl/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,som-snytt/dotty,som-snytt/dotty