Make SmallVectorImpl destructor protected (#71746)

Because the SmallVectorImpl destructor is not virtual, the destructor of
derived classes will not be called if pointers to the SmallVectorImpl
class are deleted directly. Making the SmallVectorImpl destructor
protected will prevent this.

Author: Andy Kaylor

llvm/include/llvm/ADT/SmallVector.h

@@ -601,16 +601,16 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T> {
     RHS.resetToSmall();
   }
 
-public:
-  SmallVectorImpl(const SmallVectorImpl &) = delete;
-
   ~SmallVectorImpl() {
     // Subclass has already destructed this vector's elements.
     // If this wasn't grown from the inline copy, deallocate the old space.
     if (!this->isSmall())
       free(this->begin());
   }
 
+public:
+  SmallVectorImpl(const SmallVectorImpl &) = delete;
+
   void clear() {
     this->destroy_range(this->begin(), this->end());
     this->Size = 0;