[Sema] Modernize LambdaIntroducer (NFC)

Author: kazutakahirata

clang/include/clang/Sema/DeclSpec.h

@@ -2769,11 +2769,10 @@ struct LambdaIntroducer {
 
   SourceRange Range;
   SourceLocation DefaultLoc;
-  LambdaCaptureDefault Default;
+  LambdaCaptureDefault Default = LCD_None;
   SmallVector<LambdaCapture, 4> Captures;
 
-  LambdaIntroducer()
-    : Default(LCD_None) {}
+  LambdaIntroducer() = default;
 
   bool hasLambdaCapture() const {
     return Captures.size() > 0 || Default != LCD_None;