[ Feature Request ] Move /tmp/hh_server to the repository root

**Is your feature request related to a problem? Please describe.**
When developing inside a Docker container using vscode-hack, I sometimes want to go to definition on a built-in. The /tmp/hh_server directory in the docker container is not mapped to the host machine, so this fails to load the file.

**Describe the solution you'd like**
If these definitions would live at the repository level, they would be mapped to the host machine.

**Describe alternatives you've considered**
Copying the files in /tmp/hh_server out of the container to the host. This would allow vscode to open the built-in hhi files too. Just a bit messier.

**Additional context**
Add any other context or screenshots about the feature request here.
https://github.com/hhvm/hhast/commit/e6821d522cf6c64318dbe1016cda7bb3ed383b1f commented on this before.
> it might make sense to modify the typechecker to use this subdir
instead of `/tmp/hh_server/`, which has several problems on
multi-user systems