c++: local class memfn synth from uneval context [PR113063]

c++: local class memfn synth from uneval context [PR113063]

Here we first use and therefore synthesize the local class operator<=>
from an unevaluated context, which inadvertently affects synthesization
by preventing functions used within the definition (such as the copy
constructor of std::strong_ordering) from getting marked as odr-used.

This patch fixes this by using maybe_push_to_top_level in synthesize_method
which ensures cp_unevaluated_operand gets cleared even in the function-local
case.

	PR c++/113063

gcc/cp/ChangeLog:

	* method.cc (synthesize_method): Use maybe_push_to_top_level
	and maybe_pop_from_top_level.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/spaceship-synth16.C: New test.