Add arg splat experiment initial tuple impl #212682

Job	Run time
Calculate job matrix	38s
PR - tidy	5m 24s
PR - pr-check-2	50m 20s
PR - x86_64-gnu-miri	1h 28m 24s
PR - x86_64-gnu-gcc-core-tests	17m 54s
PR - aarch64-gnu-llvm-21-1	1h 5m 50s
PR - pr-check-1	47m 5s
PR - x86_64-gnu-llvm-21	1h 22m 58s
PR - x86_64-gnu-tools	1h 6m 33s
PR - aarch64-gnu-llvm-21-2	45m 5s
PR - x86_64-gnu-next-trait-solver-polonius	1h 4m 36s
PR - x86_64-gnu-gcc	1h 16m 22s
publish toolstate	0s
	10h 11m 9s

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