[llvm] CoverageMappingWriter: Emit `Decision` before `Expansion` (PR #78966)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> To relax scanning record, tweak order by Decision < Expansion.

I think this is fine, but the rationale should be clarified.

https://github.com/llvm/llvm-project/pull/78966