HscTypes: Use foldl' instead of foldr

In this case we are building a map, for which `foldl'` is much better
suited. This has a small but consistent impact on compiler allocations,
```
        -1 s.d.                -----          -0.161%
        +1 s.d.                -----          -0.011%
        Average                -----          -0.086%
```

Test Plan: Validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2967