[clang][Interp][NFC] Implement dumping Invalid/Valid results

This was just an omission from an earlier commit, clearly
we can print them.

Author: tbaederr

clang/lib/AST/Interp/EvaluationResult.cpp

@@ -186,9 +186,12 @@ void EvaluationResult::dump() const {
     OS << "\n";
     break;
   }
-
-  default:
-    llvm_unreachable("Can't print that.");
+  case Invalid:
+    OS << "Invalid\n";
+  break;
+  case Valid:
+    OS << "Valid\n";
+  break;
   }
 }