The `dyn Trait` syntax was stabilized in 199ee327. Resolves #49277.

I think I didn't run tests properly - my second call to
select_all_obligations_or_error has made 3 tests fail. However, this is
just an error message change - integer fallback never worked with casts.

Also change several error messages to refer to "items" rather than
"methods", since associated items that require resolution during type
checking are not always methods.

remove `~self` from the test suite.

Printing <no-bounds> on trait objects comes from a time when trait
objects had a non-empty default bounds set. As they no longer have any
default bounds, printing <no-bounds> is just noise.

Some are xfailed, some not, some existing ones get un-xfailed.

The tests have consts defined both before and after their uses in order
to prevent bugs that depend on the order in which they are translated.