Stop emitting pseudomains for fatal units in HackC
Summary:
This diff turns the parse/runtime fatal pseudomains in hackc into an hhas directive.
Currently, in the assembler, we turn it back into a pseudomain. On the next diff, I'll modify the unit emitter to emit `FatalUnit*` which will be more compact than a `Unit*` and then during unit loading will emit fatal without running any code.
Reviewed By: paulbiss
Differential Revision:
D22443764
fbshipit-source-id:
a3d9557e8db91e1d899bb2d452139d0a976e1b17