[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.

lib/AST/GenericEnvironment.cpp

@@ -22,8 +22,9 @@ using namespace swift;
 
 GenericEnvironment::GenericEnvironment(
     GenericSignature *signature,
+    ArchetypeBuilder *builder,
     TypeSubstitutionMap interfaceToArchetypeMap)
-  : Signature(signature)
+  : Signature(signature), Builder(builder)
 {
   // Build a mapping in both directions, making sure to canonicalize the
   // interface type where it is used as a key, so that substitution can