[move-only] Make sure we can properly deserialize deinits from structs/enums.

We previously had a cast<ClassDecl> here. I changed it to NominalTypeDecl and
added tests for this.

rdar://102173184

Author: gottesmm

lib/Serialization/Deserialization.cpp

@@ -4491,7 +4491,7 @@ class DeclDeserializer {
 
     dtor->setGenericSignature(MF.getGenericSignature(genericSigID));
 
-    dtor->setAccess(std::max(cast<ClassDecl>(DC)->getFormalAccess(),
+    dtor->setAccess(std::max(cast<NominalTypeDecl>(DC)->getFormalAccess(),
                              AccessLevel::Internal));
 
     if (isImplicit)

test/Serialization/Inputs/moveonly_deinit.swift

@@ -0,0 +1,19 @@
+
+@_moveOnly
+public struct MoveOnlyStruct {
+  private let desc: Int
+
+  public init(desc: Int) {
+    self.desc = desc
+  }
+
+  deinit { }
+}
+
+@_moveOnly
+public enum MoveOnlyEnum {
+    case lhs(Int)
+    case rhs(Int)
+
+    deinit { }
+}

test/Serialization/moveonly_deinit.swift

@@ -0,0 +1,10 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -enable-experimental-move-only -g -emit-module  -module-name OtherModule %S/Inputs/moveonly_deinit.swift -emit-module-path %t/OtherModule.swiftmodule
+// RUN: %target-swift-frontend -enable-experimental-move-only -g -I %t %s -emit-silgen
+
+// Make sure we can deserialize deinits of both enums and structs.
+
+import OtherModule
+
+let s = MoveOnlyStruct(desc: 5)
+let e = MoveOnlyEnum.lhs(5)