libstdc++: Add returns_nonnull to non-inline std::map detail [PR108554]

libstdc++: Add returns_nonnull to non-inline std::map detail [PR108554]

std::map uses a non-inline function to rebalance its tree and the
compiler can't see that it always returns a valid pointer (assuming
valid inputs, which is a precondition anyway). This can result in
-Wnull-derefernce warnings for valid code, because the compiler thinks
there is a path where the function returns null.

Adding the returns_nonnull attribute tells the compiler that is can't
happen. While we're doing that, we might as well also add a nonnull
attribute to the rebalancing functions too.

libstdc++-v3/ChangeLog:

	PR libstdc++/108554
	* include/bits/stl_tree.h (_Rb_tree_insert_and_rebalance): Add
	nonnull attribute.
	(_Rb_tree_rebalance_for_erase): Add nonnull and returns_nonnull
	attributes.
	* testsuite/23_containers/map/modifiers/108554.cc: New test.