Access to "rustc_plugin::Registry" for CompilerControllers

[leeopop]

Currently, Registry structure is allocated and used in the rustc_driver (

rust/src/librustc_driver/driver.rs

Line 590 in 1bf5fa3

let mut registry = Registry::new(sess, &krate);

).
It is not accessible by the CompilerController.
The CompilerController can only access the read-only version of CompileState.session.plugin_attributes.
One suggestion is to manage the Registry structure inside the CompilerState structure.

I'll prepare a pull request soon to fix this issue.

@jseyfried @sanxiyn