bug#72765: Eglot + Clangd + Company + non-empty suffix = duplicate text

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 23 Aug 2024 02:07:59 +0300
> From: Dmitry Gutov <dmitry@gutov.dev>
> 
> Here's an example I came upon when testing:
> 
> 
> test.c
> ```
> int foo_bar_1;
> int foo_bar_2;
> 
> int main() {foo_bar|456
> 
> ```
> 
> Point is at |.
> 
> If you use completion-at-point, *Completions* buffer pops up, you choose 
> one of the options with M-down and M-RET, "_1" is inserted. Good.
> 
> But if you use Company, type "_" (or backspace and re-add "r") - a popup 
> comes up with "foo_bar_1" and "foo_bar_2", you choose one of the 
> options, and the text becomes "foo_bar_1456456", suffix is duplicated.
> 
> This only happens with Clang, out of the servers I've tested.
> 
> You need a fairly recent Company to reproduce (from master), the 
> previous versions simply didn't support completion in the middle of a 
> symbol.
> 
> To add something that completion-at-point trips over, though: when 
> "foo_bar_2" (existing var name) is already inserted, move point to the 
> middle of it and press C-M-i:
> 
> ```
>    foo_|bar_2
> ```
> 
> will turn to
> 
> ```
>    foo_bar_2bar_2|
> ```

Is this an Eglot problem or a completion-at-point problem?