avoid mutating input ast in `semFor` #995

Job	Run time
Provide additional context for the workflow	6s
Build release binaries (Linux)	1m 58s
Build release binaries (macOS)	3m 4s
Build source archive	0s
Build docs and release artifacts (${{ matrix.target.name }})	0s
Test the compiler and stdlib (${{ matrix.target.name }}, batch #${{ matrix.batch }})	0s
Build and test tooling (${{ matrix.target.name }})	0s
Build release artifacts from source archive (${{ matrix.target.name }})	0s
Test release artifacts	0s
All check passed	1s
	5m 9s

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