[llvm] [AIX] Fix AIX failures due to switch to internal shell (PR #160566)

[Jay Foad via llvm-commits]

jayfoad wrote:

> > Is that lit's internal echo command? What is the bug and why does it only affect AIX?
> 
> Yes, for some reason the `-n` doesn't get processed by lit's internal shell and it ends up being echo'd instead of treated like an option

That's odd. I would expect it to run via `executeBuiltinEcho` which does handle `-e` and `-n`.

https://github.com/llvm/llvm-project/pull/160566