[llvm] [Coverage] ProfileData: Handle MC/DC Bitmap as BitVector. NFC. (PR #80608)

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

@ornata Thanks for the look.

> Are there performance improvements as a result of this change?

I haven't measured yet. Since llvm-cov is still so slow with the large data, I think we have to improve it. We could measure then.

> If there are, it would be nice to include them in the commit message.

I know I haven't done completely. I guess `InstrProfReader` could decode ULEB128 into `BitVector` directly.
It was not my task for now, since it doesn't prevent my improvements.
So, it'd be happy to do when we could improve `InstrProfReader`.

https://github.com/llvm/llvm-project/pull/80608