[clang] [clang-repl] Extend the C support. (PR #89804)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> > Out of curiosity, in what context you use -fincremental-extensions?
> 
> The code snippet I provided is extracted from our internal test. We have an internal clang-tool (with the `incremental-extensions` on) to generate headers

I am a bit overwhelmed right now, are you willing to open a PR with that change so we can merge it -- otherwise I could look later this week :(

https://github.com/llvm/llvm-project/pull/89804