Summary:
Three things in this commit:

    1. Get rid of sb_ids; we are not going to use them
    to avoid infinite unfoldings in hs-boot files.

    2. Compute sb_tcs from ModIface rather than ModDetails.
    This means that the typechecker can look at this field
    without forcing the boot ModDetails, which would be
    bad if the ModDetails is not available yet (due to
    knot tying.)

    3. A big honking comment explaining what is going on
    here.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Reviewers: simonpj, austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2380