[AST] Always initialize 'TypeRepr *' in TupleTypeElement

Author: rintaro

include/swift/AST/TypeRepr.h

@@ -653,7 +653,7 @@ struct TupleTypeReprElement {
   TypeRepr *Type;
   SourceLoc TrailingCommaLoc;
 
-  TupleTypeReprElement() {}
+  TupleTypeReprElement(): Type(nullptr) {}
   TupleTypeReprElement(TypeRepr *Type): Type(Type) {}
 };