TyperState#mergeConstraintWith: use the correct context
TyperState#mergeConstraintWith: use the correct context

Previously, the code in the closure passed to `comparing` could have
been using the wrong context. I don't think this happened in practice so
far because comparingCtx was always equal to ctx but it will matter in the
next commit.

sjrd/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty