runtime: Fix overflow of swift_unownedRetain reference counts

On 32bit platforms there are 7 bits reserved for the unowned retain count. This
makes overflow a likely scenario. Implement overflow into the side table.

rdar://33495003

test/Interpreter/unowned_overflow.swift

@@ -0,0 +1,35 @@
+// RUN: %target-run-simple-swift | %FileCheck %s
+// REQUIRES: executable_test
+
+class Owner {
+  var children: [Child] = []
+
+  func addChild(_ c: Child) {
+    children.append(c)
+  }
+
+  func removeChildren() {
+    children.removeAll()
+  }
+
+  func test() {
+    // Overflow of unowned ref count on 32bit.
+    for _ in 0 ..< 500 {
+      addChild(Child(self))
+    }
+    removeChildren()
+  }
+}
+
+class Child {
+  unowned var owner: Owner
+
+  init(_ o: Owner) {
+    owner = o
+  }
+}
+
+let o = Owner()
+o.test()
+print("success")
+// CHECK: success