Insert init directive for empty product type

Fixes: #997

Sources/IR/Emitter.swift

@@ -316,6 +316,15 @@ struct Emitter {
     insertionPoint = .end(of: entry)
     let bodyFrame = Frame(locals: locals)
     let returnSite = pushing(bodyFrame, { $0.lowerStatements($0.ast[d].body!, expecting: .void) })
+
+    let receiverType: AnyType = program[d].receiver.type
+    let layout = AbstractTypeLayout(of: receiverType, definedIn: program)
+    let receiver = entry.parameter(0)
+
+    // If the object is empty, simply mark it initialized.
+    if layout.properties.isEmpty {
+      insert(module.makeMarkState(receiver, initialized: true, at: ast[d].site))
+    }
     insert(module.makeReturn(at: returnSite))
   }
 

Tests/HyloTests/TestCases/Lowering/EmptyStruct.hylo

@@ -1,5 +1,5 @@
-//- lowerToFinishedIR expecting: failure
+//- lowerToFinishedIR expecting: success
 
 public type Foo {
-  public init() { }  //! diagnostic set parameter not initialized before function returns
+  public init() { }
 }