Upgrade generated parse_line() to accept constant SBuf #3732

Job	Run time
CodeQL-tests	35m 31s
source-maintenance-tests	2m 51s
functionality-tests	17m 12s
build-tests(ubuntu-22.04,gcc,default)	10m 49s
build-tests(ubuntu-22.04,gcc,minimal)	5m 39s
build-tests(ubuntu-22.04,gcc,maximus)	12m 7s
build-tests(ubuntu-22.04,clang,default)	13m 49s
build-tests(ubuntu-22.04,clang,minimal)	7m 50s
build-tests(ubuntu-22.04,clang,maximus)	15m 45s
build-tests(macos-14,clang,default)	8m 58s
build-tests(macos-14,clang,minimal)	6m 56s
build-tests(macos-14,clang,maximus)	9m 52s
	2h 27m 19s

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