Make NameScope move constructor noexcept (#5812)

toolchain/sem_ir/name_scope.h

@@ -138,7 +138,7 @@ class NameScope : public Printable<NameScope> {
   };
 
   // Disallow copy, allow move.
-  NameScope(NameScope&& other) = default;
+  NameScope(NameScope&& other) noexcept = default;
   auto operator=(NameScope&& other) noexcept -> NameScope& = default;
 
   explicit NameScope(InstId inst_id, NameId name_id,