[IUO] Add a locator for the result of a cast to an IUO type.

Not used yet, so NFC.

lib/Sema/ConstraintLocator.h

@@ -127,6 +127,9 @@ public:
     ConditionalRequirement,
     /// A single requirement placed on the type parameters.
     TypeParameterRequirement,
+    /// \brief A coercion to an Optional type that can potentially be
+    /// force-unwrapped to the underlying type.
+    ImplicitlyUnwrappedCoercionResult,
   };
 
   /// \brief Determine the number of numeric values used for the given path
@@ -159,6 +162,7 @@ public:
     case Requirement:
     case Witness:
     case OpenedGeneric:
+    case ImplicitlyUnwrappedCoercionResult:
       return 0;
 
     case GenericArgument:
@@ -221,6 +225,7 @@ public:
     case KeyPathComponent:
     case ConditionalRequirement:
     case TypeParameterRequirement:
+    case ImplicitlyUnwrappedCoercionResult:
       return 0;
 
     case FunctionArgument: