Remove unnecessary uses of ArchetypeType::getParent().

The most interesting one of these is `ArchetypeType::getRoot()`, which
we reimplement in terms of getting the root generic parameter and then
looking up the archetype in the generic environment.

Author: DougGregor

include/swift/AST/Types.h

@@ -5734,9 +5734,6 @@ class NestedArchetypeType final : public ArchetypeType,
                       GenericEnvironment *Environment);
 };
 BEGIN_CAN_TYPE_WRAPPER(NestedArchetypeType, ArchetypeType)
-CanArchetypeType getParent() const {
-  return CanArchetypeType(getPointer()->getParent());
-}
 END_CAN_TYPE_WRAPPER(NestedArchetypeType, ArchetypeType)
 
 /// An archetype that represents an opaque element of a type sequence in context.
@@ -6615,7 +6612,7 @@ inline bool TypeBase::hasSimpleTypeRepr() const {
     return false;
 
   case TypeKind::NestedArchetype:
-    return cast<NestedArchetypeType>(this)->getParent()->hasSimpleTypeRepr();
+    return cast<NestedArchetypeType>(this)->getRoot()->hasSimpleTypeRepr();
 
   case TypeKind::OpaqueTypeArchetype:
   case TypeKind::OpenedArchetype:

lib/AST/ASTDumper.cpp

@@ -3742,7 +3742,6 @@ namespace {
     void visitNestedArchetypeType(NestedArchetypeType *T, StringRef label) {
       printArchetypeCommon(T, "nested_archetype_type", label);
       printField("name", T->getFullName());
-      printField("parent", T->getParent());
       printField("assoc_type", T->getAssocType()->printRef());
       PrintWithColorRAII(OS, ParenthesisColor) << ')';
     }

lib/AST/Type.cpp

@@ -3211,11 +3211,10 @@ ArchetypeType::ArchetypeType(TypeKind Kind,
 }
 
 ArchetypeType *ArchetypeType::getRoot() const {
-  auto parent = this;
-  while (auto nested = dyn_cast<NestedArchetypeType>(parent)) {
-    parent = nested->getParent();
-  }
-  return const_cast<ArchetypeType*>(parent);
+  auto gp = InterfaceType->getRootGenericParam();
+  assert(gp && "Missing root generic parameter?");
+  return getGenericEnvironment()->mapTypeIntoContext(
+      Type(gp))->castTo<ArchetypeType>();
 }
 
 Type ArchetypeType::getExistentialType() const {