Previously this test case would `touch %S/empty.txt` to create and empty
file. The test case then copies contents to that file, so if run a
second time the `touch` command would not create an empty file.
`llvm/utils/lit/tests/diff-test-update.py` also no longer matches against file
paths other than the final bit of the file name. This prevents test failures
on Windows due to different path separators.
This adds an updater to lit's --update-tests flag with support for
`diff`. If a RUN line containing the `diff` command fails, this function
will use heuristics to try to deduce which file is the "reference" file,
and copy the contents of the other file to the reference. If it cannot
deduce which file is the reference file, it does nothing.
The heuristics are currently:
- does one of the files end in .expected while the other does not? Then
the .expected file is the reference.
- does one of the file paths contain the substring ".tmp" while the
other does not? Then the file not containing ".tmp" is the reference.
This matches cases where one file path is constructed using the `%t`
substitution.
