Add hhconfig flag for po_enable_coeffects, clean up *.coeffects.exp and *.like_types.exp
Summary:
This flag makes it so you don't need __EnableUnstableFeatures to write down `[...]` coeffects in files, which is normally a parse error. This is the flag we'll flip for the release.
Also cleaning up old *.like_types.exp or *.coeffects.exp files that now match their .exp files
Reviewed By: DavidSnider, losvald
Differential Revision:
D25683025
fbshipit-source-id:
110217e0a83a853b1c853a389a3de5178ea3722e