[llvm] [lit] Add an option to read an xfail list from a file (PR #163959)

[David Spickett via llvm-commits]

DavidSpickett wrote:

> Python argparse supports this https://docs.python.org/3/library/argparse.html#fromfile-prefix-chars

The basic format is:
```
--option
value
```
You should experiment to see how it handles newlines or not. It could be that your 100 xfails still need to be all on one line.

https://github.com/llvm/llvm-project/pull/163959