Strip placeholders from hidden types before remapping generic parameter

[oli-obk]

When remapping generic parameters in the hidden type to the generic parameters of the definition of the opaque, we assume that placeholders cannot exist. Instead of just patching that site, I decided to handle it earlier, directly in infer_opaque_types, where we are already doing all the careful lifetime handling.

fixes #122694

the reason that ICE now occurred was that we stopped treating operation as being in the defining scope, so the TAIT became part of the hidden type of the async fn's opaque type instead of just bailing out as ambiguos

I think

use std::future::Future;

mod foo {
    type FutNothing<'a> = impl 'a + Future<Output = ()>;
    //~^ ERROR: unconstrained opaque type
}

async fn operation(_: &mut ()) -> () {
    //~^ ERROR: concrete type differs from previous
    call(operation).await
    //~^ ERROR: concrete type differs from previous
}

async fn call<F>(_f: F)
where
    for<'any> F: FnMut(&'any mut ()) -> foo::FutNothing<'any>,
{
    //~^ ERROR: expected generic lifetime parameter, found `'any`
}

would have already had the same ICE before #121796

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[compiler-errors]

r? compiler-errors @bors r+

[bors]

📌 Commit 1f3e1cf has been approved by compiler-errors

It is now in the queue for this repository.

[workingjubilee]

Broke in #122815 (comment)

[workingjubilee]

@bors r-

[oli-obk]

@bors r=compiler-errors

[bors]

📌 Commit 6623bdf has been approved by compiler-errors

It is now in the queue for this repository.