[Analyzer][solver] Fix inconsistent equivalence class data

https://bugs.llvm.org/show_bug.cgi?id=51109

When we merged two classes, `*this` became an obsolete representation of
the new `State`. This is b/c the member relations had changed during the
previous merge of another member of the same class in a way that `*this`
had no longer any members. (`mergeImpl` might keep the member relations
to `Other` and could dissolve `*this`.)

Differential Revision: https://reviews.llvm.org/D106285