Support return types in codegen specs #12039

Job	Run time
deploy_api_docs	0s
check_format	1m 33s
x86_64-gnu-test (1.7.3)	35m 38s
x86_64-gnu-test-preview_mt	5m 37s
x86_64-gnu-test (1.8.2)	34m 13s
x86_64-musl-test	38m 0s
x86_64-gnu-test (1.9.2)	33m 58s
x86_64-gnu-test (1.10.1)	33m 26s
x86_64-gnu-test (1.11.2)	34m 56s
x86_64-gnu-test (1.12.2)	34m 6s
x86_64-gnu-test (1.13.1)	35m 5s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 10s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	34m 27s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	36m 46s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	35m 45s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	35m 21s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	34m 20s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	34m 2s
	8h 49m 23s

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