[clang][NFC] Pack LambdaExpr

This saves sizeof(void *) bytes per LambdaExpr.

Review-after-commit since this is a straightforward change similar
to the work done on other nodes. NFC.
diff --git a/clang/lib/Serialization/ASTWriterStmt.cpp b/clang/lib/Serialization/ASTWriterStmt.cpp
index 45cd54f..1194132 100644
--- a/clang/lib/Serialization/ASTWriterStmt.cpp
+++ b/clang/lib/Serialization/ASTWriterStmt.cpp
@@ -1589,12 +1589,12 @@
 
 void ASTStmtWriter::VisitLambdaExpr(LambdaExpr *E) {
   VisitExpr(E);
-  Record.push_back(E->NumCaptures);
+  Record.push_back(E->LambdaExprBits.NumCaptures);
   Record.AddSourceRange(E->IntroducerRange);
-  Record.push_back(E->CaptureDefault); // FIXME: stable encoding
+  Record.push_back(E->LambdaExprBits.CaptureDefault); // FIXME: stable encoding
   Record.AddSourceLocation(E->CaptureDefaultLoc);
-  Record.push_back(E->ExplicitParams);
-  Record.push_back(E->ExplicitResultType);
+  Record.push_back(E->LambdaExprBits.ExplicitParams);
+  Record.push_back(E->LambdaExprBits.ExplicitResultType);
   Record.AddSourceLocation(E->ClosingBrace);
 
   // Add capture initializers.