[AutoFDO][llvm-profgen] Report zero count for unexecuted part of function code

In order to be consistent with compiler that interprets zero count as unexecuted(cold), this change reports zero-value count for unexecuted part of function code. For the implementation, it leverages the range counter, initializes all the executed function range with the zero-value. After all ranges are merged and converted into disjoint ranges, the remaining zero count will indicates the unexecuted(cold) part of the function.

This change also extends the current `findDisjointRanges` method which now can support adding zero-value range.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D109713