c++: implicit operator== adjustments from P2002.

c++: implicit operator== adjustments from P2002.

P2002R1, adopted at the February C++ meeting, made several refinements to
the wording for operator<=>.  This implements clarifications in how the
implicit operator== is declared: as a duplicate of the operator<=>, with
only the return type and name changed.  To that end I factored out the
declaration copying from build_clone.  For GCC 10 I'm leaving build_clone
alone, to reduce the chance of non-C++20-mode regressions.

The decl.c changes are a hack to avoid complaining about constraints on a
non-template friend that isn't defined in the class.  In this case the
defaulted comparison operator should be considered defined, but we weren't
setting funcdef_flag properly.  For GCC 11 I fixed it properly.

gcc/cp/ChangeLog:

	* cp-tree.h (copy_fndecl_with_name): Declare.
	* class.c (copy_fndecl_with_name): Copy from build_clone.
	(add_implicitly_declared_members): Add op== to TYPE_FIELDS.
	* method.c (implicitly_declare_fn): Use copy_fndecl_with_name.
	* decl.c (grokfndecl): Add initialized parm.
	(grokdeclarator): Pass it down.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/spaceship-synth9.C: New test.