[clang] [llvm] [MC/DC] Make covmap tolerant of nested Decisions (PR #125407)

[Alan Phipps via cfe-commits]

evodius96 wrote:

> @evodius96 How about this? I think this is not NFC but a simple change.
> 
> I tried this for long time locally with the Clang tree. I expect this (and this series) will be fine.

I agree, but I really want to take time to study the changes in `CoverageMapping.cpp` before signing off. I'm working on it!

https://github.com/llvm/llvm-project/pull/125407