spec: clarify that each block has its own version of iota
Issue #15550 is clearly an esoteric case but the spec was silent
about it and we had diverging implementations. By making `iota`
and index that is relative to the respective constant declaration,
nested const declarations won't affect outer values of `iota`.
cmd/compile and go/types already follow this semantics.
Fixes #15550.
Change-Id: If138189e3ea4373f8ba50ac6fb1d219b481f8698
Reviewed-on: https://go-review.googlesource.com/71750 Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Ian Lance Taylor <iant@golang.org>