Merging partial solutions can end up assigning the same type to a
particular typed node (expression, parameter, etc.), which can lead to
unbalanced set/clear when exploring the solution space (and later on,
crashes). Don't re-insert such information.
This is the same approach taken for type variable bindings, but it's
all pretty unfortunate: partial solutions should only record
information relative to their part of the constraint system, which
would save time and memory during solving. Howver, that's too big a
change for right now.
Fixes rdar://problem/50853028.
2594de613b