[test] coverage for bogus exclusivity diagnostic

We shouldn't have an exclusivity issue here.

resolves rdar://118274699

Author: kavon

test/SILOptimizer/moveonly_correct.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend %s -sil-verify-all -c -o /dev/null
+
+// This file is suppose to contain programs that are "correct" through emitting
+// an object file, so no diagnostic tests in here, please!
+
+// rdar://118274699
+struct Env: ~Copyable {
+    var constants: Int
+}
+struct VM: ~Copyable {
+    let env = Env(constants: 0)
+
+    mutating func run() {
+        env.constants
+    }
+}