Remove cgu_reuse_tracker from Session

This removes a bit of global mutable state

compiler/rustc_codegen_ssa/src/lib.rs

@@ -43,6 +43,7 @@ use std::collections::BTreeSet;
 use std::io;
 use std::path::{Path, PathBuf};
 
+pub mod assert_module_sources;
 pub mod back;
 pub mod base;
 pub mod codegen_attrs;