Rollup merge of #118000 - compiler-errors:placeholder-ty-outlives, r=aliemjay

Make regionck care about placeholders in outlives components

Currently, we don't consider a placeholder type `!T` to be a type component when it comes to processing type-outlives obligations. This means that they are essentially treated like unit values with no sub-components, and always outlive any region. This is problematic for `non_lifetime_binders`, and even more problematic for `with_negative_coherence`, since negative coherence uses placeholders as universals.

This PR adds `Component::Placeholder` which acts much like `Component::Param`. This currently causes a regression in some non-lifetime-binders tests because `for<T> T: 'static` doesn't imply itself when processing outlives obligations, so code like this will fail:

```
fn foo() where for<T> T: 'static {
  foo() //~ fails
}
```

Since the where clause doesn't imply itself. This requires making the `MatchAgainstHigherRankedOutlives` relation smarter when it comes to binders.

r? types

0 files changed, 0 insertions, 0 deletions