[GSB] Eliminate the GenericSignatureBuilder from PotentialArchetype.

Now that getBuilder() is gone, stash an ASTContext into root
PotentialArchetypes instead of a GenericSignatureBuilder. This eliminates
some ickiness where we had to re-root potential archetypes when moving a
GenericSignatureBuilder.

Author: DougGregor

include/swift/AST/GenericSignatureBuilder.h

@@ -1478,8 +1478,8 @@ struct GenericSignatureBuilder::Constraint {
 
 class GenericSignatureBuilder::PotentialArchetype {
   /// The parent of this potential archetype (for a nested type) or the
-  /// generic signature builder in which this root resides.
-  llvm::PointerUnion<PotentialArchetype*, GenericSignatureBuilder*> parentOrBuilder;
+  /// ASTContext in which the potential archetype resides.
+  llvm::PointerUnion<PotentialArchetype*, ASTContext*> parentOrContext;
 
   /// The identifier describing this particular archetype.
   ///
@@ -1545,17 +1545,16 @@ class GenericSignatureBuilder::PotentialArchetype {
 
   /// \brief Construct a new potential archetype for a concrete declaration.
   PotentialArchetype(PotentialArchetype *parent, TypeDecl *concreteDecl)
-    : parentOrBuilder(parent), identifier(concreteDecl)
+    : parentOrContext(parent), identifier(concreteDecl)
   {
     assert(parent != nullptr && "Not a nested type?");
     assert(!isa<AssociatedTypeDecl>(concreteDecl) ||
       cast<AssociatedTypeDecl>(concreteDecl)->getOverriddenDecls().empty());
   }
 
   /// \brief Construct a new potential archetype for a generic parameter.
-  PotentialArchetype(GenericSignatureBuilder *builder,
-                     GenericParamKey genericParam)
-    : parentOrBuilder(builder), identifier(genericParam)
+  PotentialArchetype(ASTContext &ctx, GenericParamKey genericParam)
+    : parentOrContext(&ctx), identifier(genericParam)
   {
   }
 
@@ -1564,13 +1563,6 @@ class GenericSignatureBuilder::PotentialArchetype {
   /// path compression on the way.
   PotentialArchetype *getRepresentative() const;
 
-private:
-  // Replace the generic signature builder.
-  void replaceBuilder(GenericSignatureBuilder *builder) {
-    assert(parentOrBuilder.is<GenericSignatureBuilder *>());
-    parentOrBuilder = builder;
-  }
-
   friend class GenericSignatureBuilder;
   friend class GenericSignature;
 
@@ -1583,7 +1575,7 @@ class GenericSignatureBuilder::PotentialArchetype {
   /// Retrieve the parent of this potential archetype, which will be non-null
   /// when this potential archetype is an associated type.
   PotentialArchetype *getParent() const { 
-    return parentOrBuilder.dyn_cast<PotentialArchetype *>();
+    return parentOrContext.dyn_cast<PotentialArchetype *>();
   }
 
   /// Retrieve the type declaration to which this nested type was resolved.
@@ -1601,7 +1593,7 @@ class GenericSignatureBuilder::PotentialArchetype {
 
   /// Determine whether this is a generic parameter.
   bool isGenericParam() const {
-    return parentOrBuilder.is<GenericSignatureBuilder *>();
+    return parentOrContext.is<ASTContext *>();
   }
 
   /// Retrieve the generic parameter key for a potential archetype that

lib/AST/GenericSignatureBuilder.cpp

@@ -2925,8 +2925,8 @@ Type GenericSignatureBuilder::PotentialArchetype::getDependentType(
 }
 
 ASTContext &PotentialArchetype::getASTContext() const {
-  if (auto builder = parentOrBuilder.dyn_cast<GenericSignatureBuilder *>())
-    return builder->getASTContext();
+  if (auto context = parentOrContext.dyn_cast<ASTContext *>())
+    return *context;
 
   return getResolvedType()->getASTContext();
 }
@@ -3068,10 +3068,6 @@ GenericSignatureBuilder::GenericSignatureBuilder(
     for (auto &gp : Impl->GenericParams) {
       gp = gp->getCanonicalType()->castTo<GenericTypeParamType>();
     }
-
-    // Point each root potential archetype at this generic signature builder.
-    for (auto pa : Impl->PotentialArchetypes)
-      pa->replaceBuilder(this);
   }
 }
 
@@ -3299,7 +3295,7 @@ void GenericSignatureBuilder::addGenericParameter(GenericTypeParamType *GenericP
 
   // Create a potential archetype for this type parameter.
   void *mem = Impl->Allocator.Allocate<PotentialArchetype>();
-  auto PA = new (mem) PotentialArchetype(this, GenericParam);
+  auto PA = new (mem) PotentialArchetype(getASTContext(), GenericParam);
   Impl->GenericParams.push_back(GenericParam);
   Impl->PotentialArchetypes.push_back(PA);
 }