[llvm] [llubi] Add support for captures (PR #201170)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Every test should have a comment explaining what it is testing. Currently this is really hard to understand and review.

I have added some comments for tests starting with `captures_mixed/nested`. I will add comments for other tests later.

> I could not figure out what the "nested" cases are trying to demonstrate.

`nested` tests demonstrate the same pointer has different provenances in different scopes. In `llvm/test/tools/llubi/captures_nested2.ll`, the provenance of the pointer captured by @mem will be updated twice after @g and @f return, so I call it `nested`.



https://github.com/llvm/llvm-project/pull/201170