We shouldn't be allocating placeholders for type
variables in the permanent arena, and we should be
caching them such that equality works.
To achieve this, we need to introduce a new
"solver allocated" type property. This is required
because we don't want to mark placeholder types
with type variable originators as themselves having
type variables, as it's not part of their structural
type. Also update ErrorType to use this bit, though
I don't believe we currently create ErrorTypes
with type variable originators.
Instead of requiring sub-classes of `ContextualMismatch` to implement
`diagnoseForAmbiguity` let's implement it directly on `ContextualMismatch`
itself and check whether all of the aggregated fixes have same types on
both sides and if so, diagnose as-if it was a single fix.
`SK_Fix` was used to indicate that solver has encountered a hole
along the current path but since there is `SK_Hole` now, increasing
`SK_Fix` no longer makes sense.
Just like in cases where both sides are dependent member types
with resolved base that can't be simplified to a concrete type
let's ignore this mismatch and mark affected type variable as a hole
because something else has to be fixed already for this to happen.
There are cases when adding missing conformance fix doesn't really
make sense like in case of `Void` and `Never` types because that
wouldn't result in a useful diagnostic.
This is a follow-up for rdar://problem/44770297
