Stop mapping opened archetypes "out of context".

These opened archetypes need to be bound within their context, so any mapping out of
the context will need to be explicit and map to newly-created generic parameters.

Author: DougGregor

lib/AST/ASTMangler.cpp

@@ -1320,10 +1320,16 @@ void ASTMangler::appendType(Type type, GenericSignature sig,
 
       // type ::= archetype
     case TypeKind::PrimaryArchetype:
-    case TypeKind::OpenedArchetype:
     case TypeKind::SequenceArchetype:
       llvm_unreachable("Cannot mangle free-standing archetypes");
 
+    case TypeKind::OpenedArchetype: {
+      // Opened archetypes have always been mangled via their interface type,
+      // although those manglings aren't used in any stable manner.
+      auto openedType = cast<OpenedArchetypeType>(tybase);
+      return appendType(openedType->getInterfaceType(), sig, forDecl);
+    }
+
     case TypeKind::OpaqueTypeArchetype: {
       auto opaqueType = cast<OpaqueTypeArchetypeType>(tybase);
       auto opaqueDecl = opaqueType->getDecl();

lib/AST/GenericEnvironment.cpp

@@ -242,7 +242,11 @@ Type MapTypeOutOfContext::operator()(SubstitutableType *type) const {
   auto archetype = cast<ArchetypeType>(type);
   if (isa<OpaqueTypeArchetypeType>(archetype->getRoot()))
     return Type();
-  
+
+  // Leave opened archetypes alone; they're handled contextually.
+  if (isa<OpenedArchetypeType>(archetype))
+    return Type(type);
+
   return archetype->getInterfaceType();
 }