add missing retainSourceCodeInfo in scalapb.gen #1871

Job	Run time
conformance	1m 51s
build (2_12, mima)	1m 26s
build (2_12, e2e)	1m 42s
build (2_12, proptest)	1m 43s
build (2_13, mima)	1m 25s
build (2_13, e2e)	1m 40s
build (2_13, proptest)	1m 43s
build (3, mima)	10s
build (3, e2e)	1m 46s
build (3, proptest)	1m 40s
build (2_12, examples_and_formatting)	1m 42s
ci-passed	0s
	16m 48s

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