The hash map mechanism that was already in use for reading swiftmodule
files on macOS is now in use also on Linux. The output replacing
mechanism has been also reworked so that:
* frontend module emission modes have the remapping done directly in
the internal frontend options instead of painstakingly modifying input
flags (this requires a patch on the swift headers though)
* object emission mode is silenced to be just a type checking pass,
thus producing no output files
* all other passes but some debugging and version related ones become
noops
The open file read redirection uses a global weak pointer instance to
maximize robustness in the face of possibly multi-threaded calls to open
happening while `main` is exiting. Possibly overkill, but better safe
than sorry.
