[llvm] [lit] Make a separate tmpdir for every test in the suite (PR #153422)

[James Henderson via llvm-commits]

jh7370 wrote:

> Thank you for the fast reply. I understand your concerns about changing this. The options you described require changing the existing testcases (and thinking about it when writing new ones). Also, if we decide to change the behavior, we can also add timestamps to temporary directories (`test.dir.1`, `test.dir.2`, ...) so that we don't have to explicitly save test artifacts before running lit again, which can be useful when, for example, debugging flaky tests etc. So, if you think the thing is not worth a discussion, please let me know so that I close the PR. Otherwise, how should I start a Discourse thread?

Most existing tests should already be using one of the two mechanisms I mentioned (and if they aren't, it's a potential test bug), so there should be no need to change them.

The LLVM Discourse forum is available at https://discourse.llvm.org/. I'd recommend posting a new thread under the `LLVM Project`, with the topic prefixed with "[RFC]". 

https://github.com/llvm/llvm-project/pull/153422