The API is pretty specific to the implementationt details of the
self-hosted compiler. I don't want to have to independently support
and maintain this as part of the standard library, and be obligated
to not make breaking changes to it with changes to the implementation of
stage2.
This is not strictly necessary but it increases the likelihood of cache
hits because foo.c and bar.c now will have different cache directories
and can be updated independently without clobbering each other's cache
data.
