[ConstraintSystem] Add a locator element to represent a generic type

This locator is going to be used as a "parent" element for
"generic argument" elements.

lib/Sema/ConstraintSystem.cpp

@@ -5372,6 +5372,7 @@ void constraints::simplifyLocator(ASTNode &anchor,
       continue;
 
    case ConstraintLocator::TupleType:
+   case ConstraintLocator::GenericType:
       path = path.slice(1);
       continue;