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

[NAKAMURA Takumi via cfe-commits]

https://github.com/chapuni commented:

They could be simpler if `SourceRegion` were compatible to `std::pair<Loc,Loc>`.
(I don't require you to work)

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