[NFC] Ensure Identifier::is() usable in debugger

Author: beccadax

include/swift/AST/Identifier.h

@@ -94,7 +94,9 @@ class Identifier {
 
   bool empty() const { return Pointer == nullptr; }
 
-  bool is(StringRef string) const { return str().equals(string); }
+  LLVM_ATTRIBUTE_USED bool is(StringRef string) const {
+    return str().equals(string);
+  }
 
   /// isOperator - Return true if this identifier is an operator, false if it is
   /// a normal identifier.