gccrs: Add ast validation check on union variant number

gccrs: Add ast validation check on union variant number

Unions with zero fields are forbidden. Add regression test for empty
unions.

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::visit): Add
	zero field check during ast validation pass.
	* checks/errors/rust-ast-validation.h: Add union visit function
	prototype.

gcc/testsuite/ChangeLog:

	* rust/compile/const_generics_8.rs: Fill the union with dummy values.
	* rust/compile/empty_union.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>