fix `defined?(@ivar)` with Ractors #55

Job	Run time
make (test, --parser=prism, -v --tty=no, 30)	4m 40s
make (test-all, --parser=prism, -q --tty=no --excludes-dir="../src/test/.excludes-prism" --exclud...	7m 49s
make (test-spec, --parser=prism, -T --parser=prism, 10)	4m 31s
make (test-tool, --parser=prism, -v --tty=no, 30)	3m 48s
make (test-bundler-parallel, --parser=prism, -v --tty=no, 30)	30m 54s
make (test-bundled-gems, --parser=prism, -v --tty=no, 30)	18m 41s
make (test-syntax-suggest, --parser=prism, -v --tty=no, 30)	3m 35s
	1h 13m 58s

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