Fix MSAN use-after-free in SmallVectorImpl dtor (#6819)

Hit when compiled with `-fsanitize-memory-use-after-dtor`. This was
fixed upstream here:

llvm/llvm-project@527352b
This applies the same patch.

Author: amaiorano

include/llvm/ADT/SmallVector.h

@@ -359,8 +359,7 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T, isPodLike<T>::value> {
 
 public:
   ~SmallVectorImpl() {
-    // Destroy the constructed elements in the vector.
-    this->destroy_range(this->begin(), this->end());
+    // Subclass has already destructed this vector's elements.
 
     // If this wasn't grown from the inline copy, deallocate the old space.
     if (!this->isSmall())
@@ -865,6 +864,11 @@ class SmallVector : public SmallVectorImpl<T> {
   SmallVector() : SmallVectorImpl<T>(N) {
   }
 
+  ~SmallVector() {
+    // Destroy the constructed elements in the vector.
+    this->destroy_range(this->begin(), this->end());
+  }
+
   explicit SmallVector(size_t Size, const T &Value = T())
     : SmallVectorImpl<T>(N) {
     this->assign(Size, Value);