Auto merge of #60441 - vext01:try-to-kill-projection-params, r=oli-obk

Make place projections concrete.

**I'm not sure if we want this. I'm raising the PR  for discussion**

Whilst doing some work on our Rust fork, I noticed the following:

Once upon a time (commit 9bd35c07c26) there were two kinds of
projection: one for places, and one for constants. It therefore made
sense to share the `Projection` struct for both. Although the different
use-cases used different concrete types, sharing was made possible by
type-parameterisation of `Projection`.

Since then, however, the usage of projections in constants has
disappeared, meaning that (forgetting lifetimes for a moment) the
parameterised type is only every instantiated under one guise. So it may
as well be a concrete type. Right?

What do people think? This is entirely untested, although it does check.

If we *don't* want this, then we should at least update the incorrect comment against `Projection`.

Thanks

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