Add type for `Enumerator::Chain` #5950

Job	Run time
test (3.1, test)	1m 5s
test (3.2, test)	1m 3s
test (3.3, test)	1m 12s
test (3.4, test)	1m 9s
test (head, test)	57s
test (head, stdlib_test rubocop)	47s
test (3.4, stdlib_test)	55s
test (3.4, test, --enable-frozen-string-literal)	1m 7s
test (3.4, stdlib_test, --enable-frozen-string-literal)	56s
test (3.4, lexer templates compile confirm_lexer confirm_templates)	2m 59s
test (3.4, rubocop validate test_doc build test_generate_stdlib raap)	1m 5s
test (3.4, typecheck_test)	1m 2s
	14m 17s

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