bug#69293: 30.0.50; cc-mode doesn't support "if consteval"

[Eli Zaretskii]

> From: Herman@debbugs.gnu.org, Géza <geza.herman@gmail.com>
> Date: Tue, 20 Feb 2024 21:21:27 +0100
> 
> 
> C++ has a new feature, "if consteval". So one can write code like
> 
> if consteval {
>     // do something
> } else {
>     // do something else
> }
> 
> The problem is that cc-mode doesn't recognize this construct, lines
> are not indented correctly: the "// do something" and the "} else {"
> receive an additional level of indentation.

Alan, could you please look into this?