[lldb] Display autosuggestion part in gray if there is one possible suggestion

I implemented autosuggestion if there is one possible suggestion.
I set the keybinds for every character. When a character is typed, Editline::TypedCharacter is called.
Then, autosuggestion part is displayed in gray, and you can actually input by typing C-k.
Editline::Autosuggest is a function for finding completion, and it is like Editline::TabCommand now, but I will add more features to it.

Testing does not work well in my environment, so I can't confirm that it goes well, sorry. I am dealing with it now.

Reviewed By: teemperor, JDevlieghere, #lldb

Differential Revision: https://reviews.llvm.org/D81001