This should make it easier to rerun crashed jobs that use filelists;
previously you'd have to run the top-level driver command again with
-save-temps. I didn't want to save /all/ temporary files because that
often includes things like .o files, which could fill up your disk
pretty quickly. But we can always tweak this later.
- Stronger CHECK lines.
- Don't try to link when there's a fake frontend.
- Give the fake 'ld' an explicit .py suffix, and use a symlink to access it, which
uncovered the first two problems.
This is only the driver side of the work; the frontend doesn't understand
this new -output-filelist option yet. Next commit.
More https://bugs.swift.org/browse/SR-280.
This is simply a newline-separated list of files, matching the format
from Darwin ld's -filelist option.
Part 2 of https://bugs.swift.org/browse/SR-280.
