Add AutoBorrowMutability; its like hir::Mutability but w/ two-phase borrow info too.

Namely, the mutable borrows also carries a flag indicating whether
they should support two-phase borrows.

This allows us to thread down, from the point of the borrow's
introduction, whether the particular adjustment that created it is one
that yields two-phase mutable borrows.

src/librustc/middle/expr_use_visitor.rs

@@ -760,7 +760,7 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
                                      expr.span,
                                      cmt_base,
                                      r,
-                                     ty::BorrowKind::from_mutbl(m),
+                                     ty::BorrowKind::from_mutbl(m.into()),
                                      AutoRef);
             }