Fix visitor-related warnings

Fix visitor-related warnings

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit):
	Fix visitor-related warnings
	* ast/rust-ast-collector.h: Likewise.
	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Likewise.
	* ast/rust-ast-visitor.h: Likewise.
	* checks/errors/borrowck/rust-bir-builder-struct.h: Likewise.
	* checks/errors/borrowck/rust-function-collector.h: Likewise.
	* checks/errors/rust-const-checker.cc (ConstChecker::visit): Likewise.
	* checks/errors/rust-const-checker.h: Likewise.
	* expand/rust-derive.h: Likewise.
	* expand/rust-macro-builtins-asm.cc (parse_reg_operand): Likewise.
	* hir/rust-hir-dump.cc (Dump::visit): Likewise.
	* hir/rust-hir-dump.h: Likewise.
	* hir/tree/rust-hir-visitor.h: Likewise.
	* resolve/rust-ast-resolve-base.cc (ResolverBase::visit): Likewise.
	* resolve/rust-ast-resolve-base.h: Likewise.