stage2: preliminary reworking for whole-file-AstGen

See #8516.

 * AstGen is now done on whole files at once rather than per Decl.

 * Introduce a new wait group for AstGen tasks. `performAllTheWork`
   waits for all AstGen tasks to be complete before doing Sema,
   single-threaded.
   - The C object compilation tasks are moved to be spawned after
     AstGen, since they only need to complete by the end of
     the function.

With this commit, the codebase compiles, but much more reworking is
needed to get things back into a useful state.

Author: andrewrk