c++: Fix static initialization from <=>.

c++: Fix static initialization from <=>.

Constant evaluation of genericize_spaceship produced a CONSTRUCTOR, which we
then wanted to bind to a reference, which we can't do.  So wrap the result
in a TARGET_EXPR so we get something with an address.

We also need to handle treating the result of cxx_eval_binary_expression as
a glvalue for SPACESHIP_EXPR.

My earlier change to add uid_sensitive to maybe_constant_value was wrong; we
don't even look at the cache when manifestly_const_eval, and I failed to
adjust the later call to cxx_eval_outermost_constant_expr.

gcc/cp/ChangeLog
2020-02-11  Jason Merrill  <jason@redhat.com>

	PR c++/93650
	PR c++/90691
	* constexpr.c (maybe_constant_value): Correct earlier change.
	(cxx_eval_binary_expression) [SPACESHIP_EXPR]: Pass lval through.
	* method.c (genericize_spaceship): Wrap result in TARGET_EXPR.