Stop telling people to submit bugs for internal feature ICEs

This keeps track of usage of internal features, and changes the message
to instead tell them that using internal features is not supported.

See MCP 620.

compiler/rustc_interface/src/tests.rs

@@ -35,6 +35,7 @@ use rustc_target::spec::{RelroLevel, SanitizerSet, SplitDebuginfo, StackProtecto
 use std::collections::{BTreeMap, BTreeSet};
 use std::num::NonZeroUsize;
 use std::path::{Path, PathBuf};
+use std::sync::Arc;
 
 type CfgSpecs = FxHashSet<(String, Option<String>)>;
 
@@ -69,6 +70,7 @@ fn mk_session(handler: &mut EarlyErrorHandler, matches: getopts::Matches) -> (Se
         None,
         "",
         None,
+        Arc::default(),
         Default::default(),
     );
     (sess, cfg)