Alec Theriault authored Jan 24, 2019 and Ben Gamari committed Jan 30, 2019

This commit relinquishes some some type information in `.hie` files in
exchange for better performance. See #16233 for more on this.

Using `.hie` files to generate hyperlinked sources is a crucial milestone
towards Hi Haddock (the initiative to move Haddock to work over `.hi`
files and embed docstrings in those). Unfortunately, even after much
optimization on the Haddock side, the `.hie` based solution is still
considerably slower and more memory hungry than the existing implementation
- and the @.hie@ code is to blame.

This changes `.hie` file generation to track type information for only
a limited subset of expressions (specifically, those that might eventually
turn into hyperlinks in the Haddock's hyperlinker backend).