[clang] fix serialization for SubstNonTypeTemplateParmPackExpr (#135428)

This fixes a PCM non-determinism regression reported here:
#134560 (comment)

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.

Author: mizvekov

clang/lib/Serialization/ASTReaderStmt.cpp

@@ -2226,10 +2226,7 @@ void ASTStmtReader::VisitSubstNonTypeTemplateParmExpr(
   E->AssociatedDeclAndRef.setPointer(readDeclAs<Decl>());
   E->AssociatedDeclAndRef.setInt(CurrentUnpackingBits->getNextBit());
   E->Index = CurrentUnpackingBits->getNextBits(/*Width=*/12);
-  if (CurrentUnpackingBits->getNextBit())
-    E->PackIndex = Record.readInt();
-  else
-    E->PackIndex = 0;
+  E->PackIndex = Record.readUnsignedOrNone().toInternalRepresentation();
   E->Final = CurrentUnpackingBits->getNextBit();
   E->SubstNonTypeTemplateParmExprBits.NameLoc = readSourceLocation();
   E->Replacement = Record.readSubExpr();
@@ -2239,6 +2236,7 @@ void ASTStmtReader::VisitSubstNonTypeTemplateParmPackExpr(
                                           SubstNonTypeTemplateParmPackExpr *E) {
   VisitExpr(E);
   E->AssociatedDecl = readDeclAs<Decl>();
+  E->Final = CurrentUnpackingBits->getNextBit();
   E->Index = Record.readInt();
   TemplateArgument ArgPack = Record.readTemplateArgument();
   if (ArgPack.getKind() != TemplateArgument::Pack)

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());