[AST] Stash an archetype builder in an "incomplete" generic environment.

This eliminates the really gross registration of archetype builders
within the ASTContext, and is another little step toward lazily
constructing archetypes.

include/swift/AST/GenericEnvironment.h

@@ -33,12 +33,20 @@ class SILType;
 /// generic parameters of a DeclContext.
 class alignas(1 << DeclAlignInBits) GenericEnvironment final {
   GenericSignature *Signature;
+  ArchetypeBuilder *Builder;
   TypeSubstitutionMap ArchetypeToInterfaceMap;
   TypeSubstitutionMap InterfaceToArchetypeMap;
 
   GenericEnvironment(GenericSignature *signature,
+                     ArchetypeBuilder *builder,
                      TypeSubstitutionMap interfaceToArchetypeMap);
 
+  friend class ArchetypeType;
+  friend class ArchetypeBuilder;
+  
+  ArchetypeBuilder *getArchetypeBuilder() const { return Builder; }
+  void clearArchetypeBuilder() { Builder = nullptr; }
+  
 public:
   GenericSignature *getGenericSignature() const {
     return Signature;
@@ -53,15 +61,14 @@ public:
   bool containsPrimaryArchetype(ArchetypeType *archetype) const;
 
   static
-  GenericEnvironment *get(ASTContext &ctx,
-                          GenericSignature *signature,
+  GenericEnvironment *get(GenericSignature *signature,
                           TypeSubstitutionMap interfaceToArchetypeMap);
 
   /// Create a new, "incomplete" generic environment that will be populated
   /// by calls to \c addMapping().
   static
-  GenericEnvironment *getIncomplete(ASTContext &ctx,
-                                    GenericSignature *signature);
+  GenericEnvironment *getIncomplete(GenericSignature *signature,
+                                    ArchetypeBuilder *builder);
 
   /// Add a mapping of a generic parameter to a specific type (which may be
   /// an archetype)