- 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.