[AST] Modernize EvalResult (NFC)

Author: kazutakahirata

clang/lib/AST/ExprConstant.cpp

@@ -12665,9 +12665,9 @@ namespace {
 class DataRecursiveIntBinOpEvaluator {
   struct EvalResult {
     APValue Val;
-    bool Failed;
+    bool Failed = false;
 
-    EvalResult() : Failed(false) { }
+    EvalResult() = default;
 
     void swap(EvalResult &RHS) {
       Val.swap(RHS.Val);