c++: Support non-type template parms of union type.

c++: Support non-type template parms of union type.

Another thing newly allowed by P1907R1.  The ABI group has discussed
representing unions with designated initializers, and has separately
specified how to represent designators; this patch implements both.

gcc/cp/ChangeLog:

	* tree.c (structural_type_p): Allow unions.
	* mangle.c (write_expression): Express unions with a designator.

libiberty/ChangeLog:

	* cp-demangle.c (cplus_demangle_operators): Add di, dx, dX.
	(d_expression_1): Handle di and dX.
	(is_designated_init, d_maybe_print_designated_init): New.
	(d_print_comp_inner): Use d_maybe_print_designated_init.
	* testsuite/demangle-expected: Add designator tests.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-class-union1.C: New test.