introduce idea of "stealable" MIR

This is a more principled version of the `RefCell` we were using
before. We now allocate a `Steal<Mir<'tcx>>` for each intermediate MIR
pass; when the next pass steals the entry, any later attempts to use it
will panic (there is no way to *test* if MIR is stolen, you're just
supposed to *know*).

src/librustc_metadata/cstore_impl.rs

@@ -30,7 +30,6 @@ use rustc::util::nodemap::{NodeSet, DefIdMap};
 use rustc_back::PanicStrategy;
 
 use std::any::Any;
-use std::mem;
 use std::rc::Rc;
 
 use syntax::ast;
@@ -102,9 +101,6 @@ provide! { <'tcx> tcx, def_id, cdata
 
         let mir = tcx.alloc_mir(mir);
 
-        // Perma-borrow MIR from extern crates to prevent mutation.
-        mem::forget(mir.borrow());
-
         mir
     }
     mir_const_qualif => { cdata.mir_const_qualif(def_id.index) }