Implement D1959R0, remove weak_equality and strong_equality.

Implement D1959R0, remove weak_equality and strong_equality.

Shortly after I finished implementing the previous semantics, the
committee decided to remove the *_equality comparison categories, because
they were largely obsoleted by the earlier change that separated operator==
from its original dependency on operator<=>.

gcc/cp/
	* method.c (enum comp_cat_tag, comp_cat_info): Remove *_equality.
	(genericize_spaceship, common_comparison_type): Likewise.
	* typeck.c (cp_build_binary_op): Move SPACESHIP_EXPR to be with the
	relational operators, exclude other types no longer supported.
libstdc++-v3/
	* libsupc++/compare: Remove strong_equality and weak_equality.

From-SVN: r277925