Add Sugar for Variadics

We used to represent the interface type of variadic parameters directly
with ArraySliceType. This was awfully convenient for the constraint
solver since it could just canonicalize and open [T] to Array<$T>
wherever it saw a variadic parameter. However, this both destroys the
sugaring of T... and locks the representation to Array<T>. In the
interest of generalizing this in the future, introduce
VariadicSequenceType. For now, it canonicalizes to Array<T> just like
the old representation. But, as you can guess, this is a new staging
point for teaching the solver how to munge variadic generic type bindings.

rdar://81628287

lib/Sema/CSGen.cpp

@@ -2623,14 +2623,13 @@ namespace {
 
       // Try to build the appropriate type for a variadic argument list of
       // the fresh element type.  If that failed, just bail out.
-      auto array = TypeChecker::getArraySliceType(expr->getLoc(), element);
-      if (array->hasError()) return element;
+      auto variadicSeq = VariadicSequenceType::get(element);
 
       // Require the operand to be convertible to the array type.
       CS.addConstraint(ConstraintKind::Conversion,
-                       CS.getType(expr->getSubExpr()), array,
+                       CS.getType(expr->getSubExpr()), variadicSeq,
                        CS.getConstraintLocator(expr));
-      return array;
+      return variadicSeq;
     }
 
     Type visitDynamicTypeExpr(DynamicTypeExpr *expr) {