Make the creation of generic environments for opaque archetype eager.

We don't need lazy creation of generic environments for opaque archetypes,
because they don't do much work up front. Make it eager, so we can
move away from delayed generic environment creation entirely.

Author: DougGregor

include/swift/AST/Types.h

@@ -5647,14 +5647,9 @@ class OpenedArchetypeType final : public ArchetypeType,
   friend TrailingObjects;
   friend ArchetypeType;
 
-  mutable GenericEnvironment *Environment = nullptr;
   TypeBase *Opened;
   UUID ID;
 
-  /// Create a generic environment with this opened type bound to its generic
-  /// parameter.
-  GenericEnvironment *createGenericEnvironment() const;
-
 public:
   /// Create a new archetype that represents the opened type
   /// of an existential value.

lib/AST/ASTContext.cpp

@@ -4319,16 +4319,13 @@ CanOpenedArchetypeType OpenedArchetypeType::get(Type existential,
       GenericTypeParamType::get(/*type sequence*/ false,
                                 /*depth*/ 0, /*index*/ 0, ctx);
 
+  auto signature = ctx.getOpenedArchetypeSignature(existential);
+  result->Environment = GenericEnvironment::forOpenedExistential(
+      signature, result);
+
   openedExistentialArchetypes[*knownID] = result;
-  return CanOpenedArchetypeType(result);
-}
 
-GenericEnvironment *OpenedArchetypeType::createGenericEnvironment() const {
-  auto thisType = const_cast<OpenedArchetypeType*>(this);
-  auto &ctx = thisType->getASTContext();
-  // Create a generic environment to represent the opened type.
-  auto signature = ctx.getOpenedArchetypeSignature(Opened);
-  return GenericEnvironment::forOpenedExistential(signature, thisType);
+  return CanOpenedArchetypeType(result);
 }
 
 CanType OpenedArchetypeType::getAny(Type existential) {

lib/AST/Type.cpp

@@ -3215,11 +3215,6 @@ GenericEnvironment *ArchetypeType::getGenericEnvironment() const {
     return Environment;
   }
 
-  // Opened archetypes lazily create their generic environment.
-  if (auto opened = dyn_cast<OpenedArchetypeType>(this)) {
-    return Environment = opened->createGenericEnvironment();
-  }
-
   // Nested archetypes get their environment from their root.
   if (auto nested = dyn_cast<NestedArchetypeType>(this)) {
     return Environment = nested->getRoot()->getGenericEnvironment();