[llvm] [coverage] keep relative order of mcdc branch and decision mapping regions (PR #91600)

[via llvm-commits]

Lambdaris wrote:

cc @evodius96 @whentojump @chapuni @ornata 
The bot seems cannot add label coverage to pr. Hope this does not bother you too much.

https://github.com/llvm/llvm-project/pull/91600