go/types, types2: don't implicitly modify an argument function's type
See the comment in the (very small) fix for a detailed description.
Use the opportunity to introduce a generic clone function which may
be useful elsewhere.
Fixes #63260.
Change-Id: Ic63c6b8bc443011b1a201908254f7d062e1aec71
Reviewed-on: https://go-review.googlesource.com/c/go/+/532157
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