Dyno: implement "is default initializable"

[DanilaFe]

This PR implements the type has default value primitive. For the most part, I attempted to mimic the approach used by production. This for the most part boils down to checking fields then invoking init(), except for some special cases like array types (which are default-initializable if their elements are) and classes (which are always default-initializable if they are nilable, and never otherwise).

There's some trickiness involved: "default initialization" sometimes involves invoking init with some actuals. Specifically, in the case of generic types, each type and param substitution gets turned into an actual to resolve the corresponding initializer. This enables resolving initializers that only require types / params that are already provided as substitutions for the type. To implement this, extract existing logic for adding substitutions as actuals from Resolver.cpp (for its use in new resolution) and share it with the new initializer call builder and existing logic in call-init-deinit.cpp

While this PR generally follows the production compiler's behavior, following a comparison with #26678 I noticed some dd behaviors. Most notably, default generic types such as numeric are treated as having a default initializer. these cases likely don't occur in practice because they are wrapped in function calls (and thus, only things that instantiate these types are actually fed to the primitive). Thus, I chose to match Anna's approach and mark those as generic.

Testing

• dyno tests

[riftEmber]

Doesn't that make sense? What value do you default init var x to?

[DanilaFe]

if it's instantiated with a type, I'd expect it to default-init to x's type's default value. Production does this, too.

[DanilaFe]

In other words, I don't see any philosophical reason why the following program shouldn't work:

record R {
  var x;
}

var x: R(int);
writeln(x);