c++: Allow floating-point template parms in C++20.

c++: Allow floating-point template parms in C++20.

P1907R1 made various adjustments to non-type template parameters, notably
introducing the notion of "structural type".  I implemented an early version
of that specification in r10-4426, but it was adjusted in the final paper to
allow more.  This patch implements allowing template parameters of
floating-point type; still to be implemented are unions and subobjects.

gcc/cp/ChangeLog:

	* pt.c (convert_nontype_argument): Handle REAL_TYPE.
	(invalid_nontype_parm_type_p): Allow all structural types.
	* tree.c (structural_type_p): Use SCALAR_TYPE_P.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/pr81246.C: No error in C++20.
	* g++.dg/cpp0x/variadic74.C: No error in C++20.
	* g++.dg/cpp1z/nontype-auto3.C: No error in C++20.
	* g++.dg/template/crash106.C: No error in C++20.
	* g++.dg/template/crash119.C: No error in C++20.
	* g++.dg/template/nontype12.C: No error in C++20.
	* g++.dg/template/void3.C: Don't require follow-on message.
	* g++.dg/template/void7.C: Don't require follow-on message.
	* g++.dg/template/void9.C: Don't require follow-on message.