gccrs: Add variadic check on function params

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::visit):
	Add variadic check on all parameters.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2850.rs: New test.

Signed-off-by: 0xn4utilus <gyanendrabanjare8@gmail.com>