Bridging: Fix typo-related bug in isValid property of swift::Identifier

`isValid` should be computed as `nonempty()`, not `empty()`.

See commit 0e0fbc4 introducing this bridge.

Author: kovdan01

include/swift/AST/Identifier.h

@@ -123,7 +123,7 @@ class Identifier {
   bool nonempty() const { return !empty(); }
 #ifdef COMPILED_WITH_SWIFT
   SWIFT_COMPUTED_PROPERTY
-  bool getIsValid() const { return empty(); }
+  bool getIsValid() const { return nonempty(); }
 #endif
 
   /// isOperator - Return true if this identifier is an operator, false if it is