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

[Jeaye Wilkerson via llvm-commits]

jeaye wrote:

@brandoncorrea Is this something we can easily reproduce with `clang-repl`? That uses the same line editor and it ships as part of LLVM. The issue never shows up in my terminal, but I know several people have seen it in theirs. If we can reproduce it in `clang-repl` then we can have a standalone repro in LLVM which doesn't rely on people knowing anything about jank.

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