[Serialization] Remove an unnecessary cast (NFC) (#146986)

Lambda is already of CXXRecordDecl *.

clang/lib/Serialization/ASTReader.cpp

diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index d380afd..6e03de8 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -9463,8 +9463,7 @@
 
   // Keep track of this lambda so it can be merged with another lambda that
   // is loaded later.
-  LambdaDeclarationsForMerging.insert(
-      {LambdaInfo, const_cast<CXXRecordDecl *>(Lambda)});
+  LambdaDeclarationsForMerging.insert({LambdaInfo, Lambda});
 }
 
 void ASTReader::LoadSelector(Selector Sel) {