[MC/DC] Refactor: Let MCDCConditionID int16_t with zero-origin (#81257)

Also, Let `NumConditions` `uint16_t`.

It is smarter to handle the ID as signed.
Narrowing to `int16_t` will reduce costs of handling byvalue. (See also
#81221 and #81227)

External behavior doesn't change. They below handle values as internal
values plus 1.
* `-dump-coverage-mapping`
* `CoverageMappingReader.cpp`
* `CoverageMappingWriter.cpp`