Disallow copy of SaveAndRestore (#7966)

An accidental copy would cause an extra save/restore. This is meant as
an RAII object to denote a save for a given scope. Also disallow move so
we don't need to add branches to the dtor.

common/checked_save_and_restore.h

@@ -12,11 +12,12 @@
 namespace Carbon {
 
 // Sets a variable to a new value, and restores the original value when
-// the SetAndRestore is destroyed. CHECKs that the variable is still holding the
-// new value when restoring the original value, to catch any data loss.
+// the CheckedSaveAndRestore is destroyed. CHECKs that the variable is still
+// holding the new value when restoring the original value, to catch any data
+// loss.
 //
 // The value must be copyable in order to compare with the new value at the time
-// the SetAndRestore is destroyed.
+// the CheckedSaveAndRestore is destroyed.
 template <typename T>
 class [[nodiscard]] CheckedSaveAndRestore {
  public:
@@ -30,6 +31,10 @@ class [[nodiscard]] CheckedSaveAndRestore {
     *var_ = std::move(orig_value_);
   }
 
+  CheckedSaveAndRestore(const CheckedSaveAndRestore&) = delete;
+  auto operator=(const CheckedSaveAndRestore&)
+      -> CheckedSaveAndRestore& = delete;
+
  private:
   T* var_;
   T new_value_;