Make rustc::middle::region::Scope's fields public...

[oli-obk]

... and remove the entire impl block for constructing values of type Scope

This will cause a lot of fallout throughout the compiler requiring changing Scope::Foo(x) to Scope { id: x, data: ScopeData::Foo }

cc #54032 (comment)

First #54032 needs to get merged though.

[maxdeviant]

@oli-obk I can take a look at this, if you'd like 👍