Add an explicit test for an invalid ManagedValue.

Author: rjmccall

lib/SILGen/ManagedValue.h

@@ -90,6 +90,7 @@ class ManagedValue {
   }
 
 public:
+  /// Constructs an invalid ManagedValue.
   ManagedValue() = default;
 
   /// Sometimes SILGen wants to represent an owned value or owned address
@@ -251,6 +252,10 @@ class ManagedValue {
     return ManagedValue(SILValue(), true, CleanupHandle::invalid());
   }
 
+  bool isValid() const {
+    return valueAndFlag.getInt() || valueAndFlag.getPointer();
+  }
+
   bool isLValue() const {
     return valueAndFlag.getInt() && valueAndFlag.getPointer();
   }