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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

<details>
<summary>Changes</summary>

`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.


---
Full diff: https://github.com/llvm/llvm-project/pull/68246.diff


1 Files Affected:

- (modified) clang/lib/AST/Interp/InterpStack.cpp (+12-7) 


``````````diff
diff --git a/clang/lib/AST/Interp/InterpStack.cpp b/clang/lib/AST/Interp/InterpStack.cpp
index da4b36f8c1bf351..b1e042c967d4241 100644
--- a/clang/lib/AST/Interp/InterpStack.cpp
+++ b/clang/lib/AST/Interp/InterpStack.cpp
@@ -85,20 +85,25 @@ 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

``````````

</details>


https://github.com/llvm/llvm-project/pull/68246