fix(exec): use STDEXEC_IMMOVABLE_NO_UNIQUE_ADDRESS in iterate operation state

The range, iterator, and sentinel members of __operation_base_base are
part of an operation state, so the plain no_unique_address attribute is
not the right macro for them: on gcc it must be dropped for immovable
types (gcc#98995). STDEXEC_IMMOVABLE_NO_UNIQUE_ADDRESS does exactly
that -- it expands to nothing on gcc and to [[no_unique_address]]
elsewhere -- and is what the rest of the codebase uses for operation
state members.

clang-format 21 clean; test.exec (394 cases) and test.stdexec (628
cases) pass in Release and Debug with STDEXEC_ENABLE_EXTRA_TYPE_CHECKING
on.

Author: alwaysprince05

include/exec/sequence/iterate.hpp

@@ -92,9 +92,11 @@ namespace experimental::execution
         }
       }
 
-      STDEXEC_ATTRIBUTE(no_unique_address) _Range __range_;
-      STDEXEC_ATTRIBUTE(no_unique_address) std::ranges::iterator_t<_Range> __iterator_;
-      STDEXEC_ATTRIBUTE(no_unique_address)
+      STDEXEC_IMMOVABLE_NO_UNIQUE_ADDRESS
+      _Range                                                                          __range_;
+      STDEXEC_IMMOVABLE_NO_UNIQUE_ADDRESS
+      std::ranges::iterator_t<_Range>                                                 __iterator_;
+      STDEXEC_IMMOVABLE_NO_UNIQUE_ADDRESS
       std::conditional_t<__cache_sentinel, std::ranges::sentinel_t<_Range>, __ignore> __sentinel_;
       trampoline_scheduler __scheduler_{};
     };