use #fileID for Swift 5.3+ (apple#187)

Motivation:

Right now, we use the full path (`#file`) for the logging as well as for
parsing out the module name (which goes into `source` by default).

This has two problems:

1. The file names contain the build server's full path, ie. aren't
   portable across different compiles (and are very long).
2. The module name parsing fails if we log from a file that's not in a
   directory named just like the module.

Modification:

Switch `#file` for `#fileID`. This changes a bunch of things:

- `source` is always the module name by default (great!)
- `file` changes from `/the/full/path/on/disk` to `ModuleName/FileBaseName.swift` (I think better, but it's a change)

Result:

`source` is more correct, `file` will be different.

Author: weissi