c++: Allow parm of empty class type in constexpr.

c++: Allow parm of empty class type in constexpr.

Since copying a class object is defined in terms of the copy constructor,
copying an empty class is OK even if it would otherwise not be usable in a
constant expression.  Relatedly, using a parameter as an lvalue is no more
problematic than a local variable, and calling a member function uses the
object as an lvalue.

gcc/cp/ChangeLog
2020-03-02  Jason Merrill  <jason@redhat.com>

	PR c++/91953
	* constexpr.c (potential_constant_expression_1) [PARM_DECL]: Allow
	empty class type.
	[COMPONENT_REF]: A member function reference doesn't use the object
	as an rvalue.