Move rustc_middle::limits to rustc_interface.

It's always good to make `rustc_middle` smaller. `rustc_interface` is
the best destination, because it's the only crate that calls
`get_recursive_limit`.

compiler/rustc_interface/src/lib.rs

@@ -10,6 +10,7 @@
 mod callbacks;
 pub mod errors;
 pub mod interface;
+mod limits;
 pub mod passes;
 mod proc_macro_decls;
 mod queries;