[llvm] Revert "[MC/DC] Make covmap tolerant of nested Decisions (#125407)" (PR #181069)

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

@gulfemsavrun I cannot download two files in the URL (500). Seems small files could be downloaded.

I can wait, for a few days, for the content server would get back better but I'd be happy if you (or anyone else) would send me files via other paths.

This revert has broken functionality of "MC/DC nested expressions".

https://github.com/llvm/llvm-project/pull/181069