Summary:
This makes a big performance difference especially when loading a
large number of modules and using parallel compilation (ghci -jN).

Test Plan:
* validate
* `ghci -fexternal-interpreter` in `nofib/real/anna`

Reviewers: niteria, bgamari, ezyang, austin, hvr, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1876

GHC Trac Issues: #11100