[clang] [clang] Allow trivial pp-directives before C++ module directive (PR #153641)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> Thanks for fixing this. Will the fix end up in version 21?

I think we need to backport this.

https://github.com/llvm/llvm-project/pull/153641