[clang] [Coverage][Expansion] handle nested macros in scratch space (PR #89869)

[Wentao Zhang via cfe-commits]

whentojump wrote:

Thanks everyone!

I notice that #87000 has been tagged with release milestone. I can handle the backport there. Maybe in a few days? Or sooner if that's needed.

https://github.com/llvm/llvm-project/pull/89869