Make Compiler::{sess,codegen_backend} public.

And remove the relevant getters on `Compiler` and `Queries`.

src/librustdoc/lib.rs

@@ -794,7 +794,7 @@ fn main_args(
     let config = core::create_config(options, &render_options, using_internal_features);
 
     interface::run_compiler(config, |compiler| {
-        let sess = compiler.session();
+        let sess = &compiler.sess;
 
         if sess.opts.describe_lints {
             rustc_driver::describe_lints(sess);