Since `ConstraintFix` references `ConstraintSystem` directly now,
we can get `SourceManager` from `ASTContext` associated with that
`ConstraintSystem` instead of passing it in every time.
There is no longer any reason to attach solution to `ConstraintFix`
because solution applied to the constraint system before any of the
related fixes are diagnosed, so instead let's attach `ConstraintSystem`
to `ConstraintFix` directly, because it would have all of the required
information.
Since constraint fix life span is tightly coupled with particular
constraint system, it makes sense to allocate fixes using the same
allocator as used for constraints.
Make is so `FixKind` lives in the new `CSFix` header and
all new `ConstraintFix` classes get a kind. That's useful
when trying to identify if locator has multiple fixes of
the same kind attached to it.
A fix is related to one of the constraints through its locator,
and contains information required to "fix" a failure associated with
given constraint, each of the fixes also includes diagnostic.
