libstdc++: Avoid -Wmaybe-uninitialized warning in std::stop_source [PR109339]

libstdc++: Avoid -Wmaybe-uninitialized warning in std::stop_source [PR109339]

We pass a const-reference to *this before it's constructed, and GCC
assumes that all const-references are accessed. Add the access attribute
to say it's not accessed.

libstdc++-v3/ChangeLog:

	PR libstdc++/109339
	* include/std/stop_token (_Stop_state_ptr(const stop_source&)):
	Add attribute access with access-mode 'none'.
	* testsuite/30_threads/stop_token/stop_source/109339.cc: New test.