Invert `seen_${var}` to `${var}_seen` (#2789)

## Motivation and Context

When `var` is a reserved word, `seen_${var}` becomes `seen_r#var` which
is invalid Rust.

## Description

Invert `seen_${var}` to `${var}_seen`. This is a least effort fix.

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Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>
Co-authored-by: 82marbag <69267416+82marbag@users.noreply.github.com>