[llvm] Turn signal handling on in LineEditor (PR #203616)

[Jeaye Wilkerson via llvm-commits]

jeaye wrote:

> I could not find tests for the rest either, do you think it would be a lot of work to add some testing infrastructure around?

Vassil, I think it this is particularly difficult case to test. The issue is only observed because the line editor leaves the user's terminal in a bad state after receiving a signal to quit. The issue is then visible to the user, when opening a *different* application which also does line editing.

This is not something we can represent with a unit test and it's something which would be possible but wholly different to try to encase in an integration test.

More info is available here, by searching for `EL_SIGNAL`: https://linux.die.net/man/3/el_set

It's clear from the docs that setting `EL_SIGNAL` will also improve the handling of other cases, such as terminal resizing.

https://github.com/llvm/llvm-project/pull/203616