Many, many, many types in the Swift compiler are intended to only be allocated in the ASTContext. We have previously implemented this by writing several `operator new` and `operator delete` implementations into these types. Factor those out into a new base class instead.
This silences the instances of the warning from Visual Studio about not all
codepaths returning a value. This makes the output more readable and less
likely to lose useful warnings. NFC.
This PR addresses TODOs from #8241.
- It supports merging for layout constraints, e.g., if both a _Trivial constraint and a _Trivial(64) constraint appear on a type parameter, we keep only _Trivial(64) as a more specific layout constraint. We do a similar thing for ref-counted/native-ref-counted. The overall idea is to keep the more specific of two compatible layout constraints.
- The presence of a superclass constraint implies a layout constraint, e.g., a superclass constraint implies _Class or _NativeClass
- Add support for _Class and _NativeClass layout constraints, which are supposed to represent T: Superclass and P: class constraints in the future.
- Use the re-factoring to also reduce the number of dynamic allocations when creating layout constraints. Simple non-parametrized layout constraints are now represented as statically allocated singletons (static members of LayoutConstraintInfo).
This biggest change is:
- LayoutConstraintInfo is now a FoldingSetNode, which allows for proper canonicalization of LayoutConstraints. This is important for the correctness of type comparisons if types contain layout constraints.
No functionality changes from the client's point of view.
