Bug#1124039: iwyu: llvm/clang 21

[Philipp Matthias Hahn]

Hello,

just changing the build dependency is *not* enough.
The code itself must be updated to be compatible with llvm/clang 21.

Luckily upstream already did this in 0.25:
<https://github.com/include-what-you-use/include-what-you-use/releases/tag/0.25>
> Compatible with Clang 21.

The already is yet a newer version 0.26:
<https://github.com/include-what-you-use/include-what-you-use/releases/tag/0.26>
> Compatible with Clang 22.

Philipp