Remove ImplicitlyUnwrappedOptionalType.

We haven't been creating these since
https://github.com/apple/swift/pull/14299 but there was more
groundwork before we could actually remove the type definition.

include/swift/AST/TypeNodes.def

@@ -150,8 +150,7 @@ ABSTRACT_SUGARED_TYPE(Sugar, Type)
     ABSTRACT_SUGARED_TYPE(UnarySyntaxSugar, SyntaxSugarType)
       SUGARED_TYPE(ArraySlice, UnarySyntaxSugarType)
       SUGARED_TYPE(Optional, UnarySyntaxSugarType)
-      SUGARED_TYPE(ImplicitlyUnwrappedOptional, UnarySyntaxSugarType)
-      TYPE_RANGE(UnarySyntaxSugar, ArraySlice, ImplicitlyUnwrappedOptional)
+      TYPE_RANGE(UnarySyntaxSugar, ArraySlice, Optional)
     SUGARED_TYPE(Dictionary, SyntaxSugarType)
     TYPE_RANGE(SyntaxSugar, ArraySlice, Dictionary)
   TYPE_RANGE(Sugar, Paren, Dictionary)

include/swift/AST/Types.h

@@ -4102,26 +4102,6 @@ public:
   }
 };
 
-/// The type T!, which is always sugar for a library type.
-class ImplicitlyUnwrappedOptionalType : public UnarySyntaxSugarType {
-  ImplicitlyUnwrappedOptionalType(const ASTContext &ctx, Type base,
-                        RecursiveTypeProperties properties)
-    : UnarySyntaxSugarType(TypeKind::ImplicitlyUnwrappedOptional, ctx, base,
-                           properties) {
-    llvm_unreachable(
-        "ImplicitlyUnwrappedOptionalType::ImplicitlyUnwrappedOptionalType");
-  }
-
-public:
-  /// Return a uniqued optional type with the specified base type.
-  static ImplicitlyUnwrappedOptionalType *get(Type baseTy);
-
-  // Implement isa/cast/dyncast/etc.
-  static bool classof(const TypeBase *T) {
-    return T->getKind() == TypeKind::ImplicitlyUnwrappedOptional;
-  }
-};
-
 /// The dictionary type [K : V], which is syntactic sugar for Dictionary<K, V>.
 ///
 /// Example: