Fix the return type of `Enumerable#sum` for union elements #13107

Job	Run time
check_format	1m 41s
x86_64-gnu-test-preview_mt	6m 41s
x86_64-gnu-test (1.7.3)	34m 23s
x86_64-musl-test	38m 6s
x86_64-gnu-test (1.8.2)	37m 29s
x86_64-gnu-test (1.9.2)	38m 39s
x86_64-gnu-test (1.10.1)	35m 37s
x86_64-gnu-test (1.11.2)	36m 19s
x86_64-gnu-test (1.12.2)	35m 59s
x86_64-gnu-test (1.13.3)	34m 53s
x86_64-gnu-test (1.14.0)	35m 15s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 31s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	34m 21s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	35m 12s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	34m 45s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	34m 46s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	35m 37s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	34m 11s
	9h 36m 25s

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