[Archetype builder] Stub out getNestedType() for associated types. NFC

Start using this entrypoint when we have a specific associated
type. The eventual goal here is to create the invariant

  resolveArchetype(t)->getDependentType()->isEqual(t)

when t has fully-resolved DependentMemberTypes.

Author: DougGregor

include/swift/AST/ArchetypeBuilder.h

@@ -574,6 +574,9 @@ class ArchetypeBuilder::PotentialArchetype {
   PotentialArchetype *getNestedType(Identifier Name,
                                     ArchetypeBuilder &builder);
 
+  /// \brief Retrieve (or create) a nested type with a known associated type.
+  PotentialArchetype *getNestedType(AssociatedTypeDecl *assocType,
+                                    ArchetypeBuilder &builder);
 
   /// \brief Retrieve (or build) the type corresponding to the potential
   /// archetype within the given generic environment.

lib/AST/ArchetypeBuilder.cpp

@@ -569,6 +569,12 @@ auto ArchetypeBuilder::PotentialArchetype::getNestedType(
   return nested.front();
 }
 
+auto ArchetypeBuilder::PotentialArchetype::getNestedType(
+                            AssociatedTypeDecl *assocType,
+                            ArchetypeBuilder &builder) -> PotentialArchetype * {
+  return getNestedType(assocType->getName(), builder);
+}
+
 Type ArchetypeBuilder::PotentialArchetype::getTypeInContext(
                                                ArchetypeBuilder &builder,
                                                GenericEnvironment *genericEnv) {
@@ -913,6 +919,9 @@ auto ArchetypeBuilder::resolveArchetype(Type type) -> PotentialArchetype * {
     if (!base)
       return nullptr;
 
+    if (auto assocType = dependentMember->getAssocType())
+      return base->getNestedType(assocType, *this);
+
     return base->getNestedType(dependentMember->getName(), *this);
   }
 
@@ -1003,7 +1012,7 @@ bool ArchetypeBuilder::addConformanceRequirement(PotentialArchetype *PAT,
   for (auto Member : Proto->getMembers()) {
     if (auto AssocType = dyn_cast<AssociatedTypeDecl>(Member)) {
       // Add requirements placed directly on this associated type.
-      auto AssocPA = T->getNestedType(AssocType->getName(), *this);
+      auto AssocPA = T->getNestedType(AssocType, *this);
       if (AssocPA != T) {
         if (addAbstractTypeParamRequirements(AssocType, AssocPA,
                                              RequirementSource::Protocol,

lib/Sema/TypeCheckGeneric.cpp

@@ -55,7 +55,7 @@ Type DependentGenericTypeResolver::resolveSelfAssociatedType(
   assert(archetype && "Bad generic context nesting?");
 
   return archetype
-           ->getNestedType(assocType->getName(), Builder)
+           ->getNestedType(assocType, Builder)
            ->getDependentType(/*FIXME: */{ }, /*allowUnresolved=*/true);
 }
 
@@ -214,7 +214,7 @@ Type CompleteGenericTypeResolver::resolveDependentMemberType(
 Type CompleteGenericTypeResolver::resolveSelfAssociatedType(
        Type selfTy, AssociatedTypeDecl *assocType) {
   return Builder.resolveArchetype(selfTy)
-           ->getNestedType(assocType->getName(), Builder)
+           ->getNestedType(assocType, Builder)
            ->getDependentType(/*FIXME: */{ }, /*allowUnresolved=*/false);
 }