c++: Reject float <=> enum.

c++: Reject float <=> enum.

As [depr.arith.conv.enum] says, these are ill-formed.

gcc/cp/ChangeLog:

	* typeck.c (do_warn_enum_conversions): Don't warn for SPACESHIP_EXPR.
	(cp_build_binary_op): Reject float <=> enum or enum <=> float.  Use
	CP_INTEGRAL_TYPE_P instead of INTEGRAL_OR_ENUMERATION_TYPE_P.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/enum-conv1.C: Remove unused code.
	* g++.dg/cpp2a/spaceship-err5.C: New test.