This makes the difference between errors and lower-level diagnostics
visible to users, and aligns the toolchain's behavior with the
expectations in `driver_fuzzer.cpp`.
toolchain/driver/testdata/fail_missing_file.carbon has no content after
its AUTOUPDATE line. This caused it to oscillate between three states:
1) If there were no content lines after AUTOUPDATE, autoupdate would add
a blank line and then its STDERR CHECK line.
2) If there was a (blank) content line after AUTOUPDATE, autoupdate
would move the STDERR CHECK line to immediately after AUTOUPDATE,
leaving a trailing blank line.
3) pre-commit would remove the trailing blank line, leaving no content
lines after AUTOUPDATE.
Handle the special case of no content lines after AUTOUPDATE by
producing "early" STDERR check lines immediately after the AUTOUPDATE
line, rather than before the next line, which might not exist. We
already did this in the case where there were early STDOUT check lines.
This updates SourceBuffer to diagnostics. Some additional edits to
diagnostics were necessary due to issues moving arguments around, which
seems to stem from a compile error with clang 14 (fixed in later
versions).
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Jon Ross-Perkins
Geoff Romer
Richard Smith