This is a fix related to D135414. The original intention was to keep `BaseFS` as a member of the worker and conditionally overlay it with local in-memory FS. The `move` of ref-counted `BaseFS` was not intended, and it's a bug.
Disabling parallelism in the "by-module-name" test reliably reproduces this, and the test itself doesn't *need* parallelism. (I think `-j 4` was cargo culted from another test.) Reusing that test to check for correct behavior...
Reviewed By: DavidSpickett
Differential Revision: https://reviews.llvm.org/D136124
d239f3c627