[clang] fix serialization for SubstNonTypeTemplateParmPackExpr (#135428)

This fixes a PCM non-determinism regression reported here:
https://github.com/llvm/llvm-project/pull/134560#issuecomment-2797744370

There was a bit in `SubstNonTypeTemplateParmPackExpr` which we missed to
serialize, and that bit eventually propagates to
`SubstNonTypeTemplateParmExpr`.

As a drive by, improve serialization for PackIndex on
SubstNonTypeTemplateParmExpr by using the newly introduced
UnsignedOrNone helpers.

There are no release notes since this regression was never released.

1 files changed, 2 insertions, 3 deletions

diff --git a/clang/lib/Serialization/ASTWriterStmt.cpp b/clang/lib/Serialization/ASTWriterStmt.cpp
index 939b37c..b9eabd5 100644
--- a/clang/lib/Serialization/ASTWriterStmt.cpp
+++ b/clang/lib/Serialization/ASTWriterStmt.cpp
@@ -2228,9 +2228,7 @@ void ASTStmtWriter::VisitSubstNonTypeTemplateParmExpr(
   Record.AddDeclRef(E->getAssociatedDecl());
   CurrentPackingBits.addBit(E->isReferenceParameter());
   CurrentPackingBits.addBits(E->getIndex(), /*Width=*/12);
-  CurrentPackingBits.addBit((bool)E->getPackIndex());
-  if (auto PackIndex = E->getPackIndex())
-    Record.push_back(*PackIndex + 1);
+  Record.writeUnsignedOrNone(E->getPackIndex());
   CurrentPackingBits.addBit(E->getFinal());
 
   Record.AddSourceLocation(E->getNameLoc());
@@ -2242,6 +2240,7 @@ void ASTStmtWriter::VisitSubstNonTypeTemplateParmPackExpr(
                                           SubstNonTypeTemplateParmPackExpr *E) {
   VisitExpr(E);
   Record.AddDeclRef(E->getAssociatedDecl());
+  CurrentPackingBits.addBit(E->getFinal());
   Record.push_back(E->getIndex());
   Record.AddTemplateArgument(E->getArgumentPack());
   Record.AddSourceLocation(E->getParameterPackLocation());