cbuilder: upper half of cgen, variable decls (#24423) #27991

Job	Run time
ubuntu-20.04 (batch: allowed_failures)	4m 33s
ubuntu-20.04 (batch: 0_3)	23m 29s
ubuntu-20.04 (batch: 1_3)	28m 50s
ubuntu-20.04 (batch: 2_3)	22m 59s
macos-13 (batch: allowed_failures)	7m 1s
macos-13 (batch: 0_3)	28m 27s
macos-13 (batch: 1_3)	34m 10s
macos-13 (batch: 2_3)	29m 25s
	2h 58m 54s

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