c++: Don't synthesize sfk_comparison method multiple times [PR94907]

c++: Don't synthesize sfk_comparison method multiple times [PR94907]

On the following testcase we ICE, because synthesize_method is called twice
on the same sfk_comparison method fndecl, the first time it works fine
because start_preparsed_function in that case sets both
current_function_decl and cfun, but second time it is called it only sets
the former and keeps cfun NULL, so we ICE when trying to store
current_function_returns_value.
I think it is just wrong to call synthesize_method multiple times, and most
synthesize_method callers avoid that by not calling it if DECL_INITIAL is
already set, so this patch does that too.

2020-05-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94907
	* method.c (defaulted_late_check): Don't call synthesize_method
	on constexpr sfk_comparison if it has been called on it already.

	* g++.dg/cpp2a/spaceship-synth8.C: New test.