libstdc++: Add self-merge check to std::forward_list::merge [PR103853]

libstdc++: Add self-merge check to std::forward_list::merge [PR103853]

This implements the proposed resolution of LWG 3088, so that x.merge(x)
is a no-op, consistent with std::list::merge.

Signed-off-by: Pavel I. Kryukov <pavel.kryukov@phystech.edu>
Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	PR libstdc++/103853
	* include/bits/forward_list.tcc (forward_list::merge): Check for
	self-merge.
	* testsuite/23_containers/forward_list/operations/merge.cc: New test.