libstdc++: Micro-optimization for std::optional [PR112480]

This small change removes a branch when clearing a std::optional<T> for
types with no-op destructors. For types where the destructor can be
optimized away (e.g. because it's trivial, or empty and can be inlined)
the _M_destroy() function does nothing but set _M_engaged to false.
Setting _M_engaged=false unconditionally is cheaper than only doing it
when initially true, because it allows the compiler to remove a branch.

The compiler thinks it would be incorrect to unconditionally introduce a
store there, because it could conflict with reads in other threads, so
it won't do that optimization itself. We know it's safe to do because
we're in a non-const member function, so the standard forbids any
potentially concurrent calls to other member functions of the same
object. Making the store unconditional can't create a data race that
isn't already present in the program.

libstdc++-v3/ChangeLog:

	PR libstdc++/112480
	* include/std/optional (_Optional_payload_base::_M_reset): Set
	_M_engaged to false unconditionally.

libstdc++-v3/include/std/optional

@@ -311,6 +311,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	if (this->_M_engaged)
 	  _M_destroy();
+	else // This seems redundant but improves codegen, see PR 112480.
+	  this->_M_engaged = false;
       }
     };