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

* `getFunctionBitmap()` stores not `std::vector<uint8_t>` but
`BitVector`.
* `CounterMappingContext` holds `Bitmap` (instead of the ref of bytes)
* `Bitmap` and `BitmapIdx` are used instead of `evaluateBitmap()`.

FIXME: `InstrProfRecord` itself should handle `Bitmap` as `BitVector`.