[clang][Interp] Fix crash during InterpStack printing (#68246)

`InterpStack` is using an `std::vector<>` to track the `ItemTypes`. As a
result, the new types are inserted
to the back of the `std::vector<>`, however `dump()` was reading the
types from the front (the bottom
of the stack) and printing the value on the top of the stack.

This lead to a crash if the type on the bottom had a different type from
the type on the top. E.g.:
```
Items: 2. Size: 40
0/8: 0
1/40: 0x5590cddc0460 {16, 16, 32}
```

The same method also miscalculated the offsets during printing the
stack, which was a source of
incorrect stack dumps and future crashes.

This patch changes the order of iteration of the types and fixes the
offset calculation.

As for testing the change, the issue is that it needs to be done as a
unittest, however from
`clang/unittests` we don't have access to `clang/lib`, where `Interp`
resides. Although the previous
implementation didn't have unittests either, so I'm not sure if we
actually care that much or not.

clang/lib/AST/Interp/InterpStack.cpp

@@ -86,20 +86,25 @@ void InterpStack::shrink(size_t Size) {
 
 void InterpStack::dump() const {
 #ifndef NDEBUG
-  llvm::errs() << "Items: " << ItemTypes.size() << ". Size: " << size() << "\n";
+  llvm::errs() << "Items: " << ItemTypes.size() << ". Size: " << size() << '\n';
   if (ItemTypes.empty())
     return;
 
   size_t Index = 0;
-  size_t Offset = align(primSize(ItemTypes[0]));
-  for (PrimType Ty : ItemTypes) {
-    llvm::errs() << Index << "/" << Offset << ": ";
-    TYPE_SWITCH(Ty, {
+  size_t Offset = 0;
+
+  // The type of the item on the top of the stack is inserted to the back
+  // of the vector, so the iteration has to happen backwards.
+  for (auto TyIt = ItemTypes.rbegin(); TyIt != ItemTypes.rend(); ++TyIt) {
+    Offset += align(primSize(*TyIt));
+
+    llvm::errs() << Index << '/' << Offset << ": ";
+    TYPE_SWITCH(*TyIt, {
       const T &V = peek<T>(Offset);
       llvm::errs() << V;
     });
-    llvm::errs() << "\n";
-    Offset += align(primSize(Ty));
+    llvm::errs() << '\n';
+
     ++Index;
   }
 #endif