[clang][bytecode] Keep the last chunk in InterpStack::clear() (#144487)

We call clear when checking for potential constant expressions, but that
used to free all the chunks. Keep the last one so we don't have to
re-allocate it.

clang/lib/AST/ByteCode/InterpStack.cpp

@@ -19,9 +19,7 @@
 using namespace clang;
 using namespace clang::interp;
 
-InterpStack::~InterpStack() { clear(); }
-
-void InterpStack::clear() {
+InterpStack::~InterpStack() {
   if (Chunk && Chunk->Next)
     std::free(Chunk->Next);
   if (Chunk)
@@ -33,6 +31,21 @@ void InterpStack::clear() {
 #endif
 }
 
+// We keep the last chunk around to reuse.
+void InterpStack::clear() {
+  if (!Chunk)
+    return;
+
+  if (Chunk->Next)
+    std::free(Chunk->Next);
+
+  assert(Chunk);
+  StackSize = 0;
+#ifndef NDEBUG
+  ItemTypes.clear();
+#endif
+}
+
 void InterpStack::clearTo(size_t NewSize) {
   assert(NewSize <= size());
   size_t ToShrink = size() - NewSize;