Label `non_exhaustive` attribute on privacy errors from non-local items
Label when an ADT is `non_exhaustive` and we get a privacy error, help with confusion in a case like this:
```rust
#[non_exhaustive]
pub struct Foo;
// other crate
let x = Foo;
//~^ ERROR unit struct `Foo` is private
```
Check pattern refutability on THIR
The current `check_match` query is based on HIR, but partially re-lowers HIR into THIR.
This PR proposed to use the results of the `thir_body` query to check matches, instead of re-building THIR.
Most of the diagnostic changes are spans getting shorter, or commas/semicolons not getting removed.
This PR degrades the diagnostic for confusing constants in patterns (`let A = foo()` where `A` resolves to a `const A` somewhere): it does not point ot the definition of `const A` any more.
