Added swift_retainCount to EmbeddedRuntime.swift

Fixes test/embedded/concurrency-actors.swift

include/swift/Runtime/RuntimeFunctions.def

@@ -204,6 +204,14 @@ FUNCTION(CopyPOD, swift_copyPOD, C_CC, AlwaysAvailable,
          EFFECT(NoEffect),
          UNKNOWN_MEMEFFECTS)
 
+// size_t swift_retainCount(void *ptr);
+FUNCTION(NativeRetainCount, swift_retainCount, C_CC, AlwaysAvailable,
+         RETURNS(SizeTy),
+         ARGS(RefCountedPtrTy),
+         ATTRS(NoUnwind, WillReturn),
+         EFFECT(RefCounting),
+         UNKNOWN_MEMEFFECTS)
+
 // void *swift_retain(void *ptr);
 FUNCTION(NativeStrongRetain, swift_retain, C_CC, AlwaysAvailable,
          RETURNS(RefCountedPtrTy),

stdlib/public/core/EmbeddedRuntime.swift

@@ -309,7 +309,13 @@ public func swift_bridgeObjectRelease_n(object: Builtin.RawPointer, n: UInt32) {
   swift_release_n(object: untaggedObject, n: n)
 }
 
-
+@_cdecl("swift_retainCount")
+public func swift_retainCount(object: Builtin.RawPointer) -> Int {
+  if !isValidPointerForNativeRetain(object: object) { return 0 }
+  let o = UnsafeMutablePointer<HeapObject>(object)
+  let refcount = refcountPointer(for: o)
+  return loadAcquire(refcount) & HeapObject.refcountMask
+}
 
 /// Refcount helpers