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

[Haojian Wu via cfe-commits]

hokein 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 

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