Destroy locals at the end of blocks, not only on return (#7448)

Destroy local variables and temporaries at each `}`, and when branching
with `break` and `continue`. In `for` statements, destroy loop variables
along with anything created within the loop at the end of each loop
iteration, and destroy the cursor and range object when the loop
terminates.

Assisted-by: Gemini via Antigravity

---------

Co-authored-by: Geoff Romer <gromer@google.com>

common/array_stack.h

@@ -86,6 +86,24 @@ class ArrayStack {
     llvm::append_range(values_, values);
   }
 
+  // Truncates the top array on the stack to the given size.
+  auto TruncateTopArray(size_t size) -> void {
+    CARBON_CHECK(!array_offsets_.empty());
+    values_.truncate(array_offsets_.back() + size);
+  }
+
+  // Merges the top array into the the array two before it, leaving the top
+  // array empty.
+  auto MergeTopArrayIntoGrandparent() -> void {
+    CARBON_CHECK(array_offsets_.size() >= 3);
+    auto new_mid =
+        std::rotate(values_.begin() + array_offsets_[array_offsets_.size() - 2],
+                    values_.begin() + array_offsets_[array_offsets_.size() - 1],
+                    values_.end());
+    array_offsets_[array_offsets_.size() - 2] = new_mid - values_.begin();
+    array_offsets_[array_offsets_.size() - 1] = values_.size();
+  }
+
   // Returns the current number of values in all arrays.
   auto all_values_size() const -> size_t { return values_.size(); }