[embedded] Add swift_allocEmptyBox to the embedded runtime

Author: kubamracek

stdlib/public/core/EmbeddedRuntime.swift

@@ -236,6 +236,15 @@ func swift_initStackObject(metadata: UnsafeMutablePointer<ClassMetadata>, object
   return unsafe object
 }
 
+@unsafe
+public var _emptyBoxStorage: (Int, Int) = (/*isa*/0, /*refcount*/-1)
+
+@_cdecl("swift_allocEmptyBox")
+public func swift_allocEmptyBox() -> Builtin.RawPointer {
+  let box = unsafe Builtin.addressof(&_emptyBoxStorage)
+  swift_retain(object: box)
+  return box
+}
 
 
 /// Refcounting

test/embedded/empty-box.swift

@@ -0,0 +1,24 @@
+// RUN: %target-run-simple-swift(-enable-experimental-feature Embedded -wmo) | %FileCheck %s
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: executable_test
+// REQUIRES: swift_feature_Embedded
+
+public struct ZeroSizedStruct {}
+
+var escape: (()->())? = nil
+
+public func sink<T>(t: inout T) {
+
+}
+
+public func foo() {
+    var s = ZeroSizedStruct() 
+    escape = {
+        sink(t: &s)
+    }
+}
+
+foo()
+print("OK!")
+// CHECK: OK!