[lldb][progress] Correctly check total for deterministic progress (#79912)

The `total` parameter for the constructor for Progress was changed to a
std::optional in https://github.com/llvm/llvm-project/pull/77547. It was originally set to 1 to indicate non-determinisitic progress, but this commit changes this. First, `UINT64_MAX` will again be used for non-deterministic progress, and `Progress` now has a static variable set to this value so that we can use this instead of a magic number. 

The member variable `m_total` could be changed to a std::optional as
well, but this means that the `ProgressEventData::GetTotal()` (which is
used for the public API) would
either need to return a std::optional value or it would return some
specific integer to represent non-deterministic progress if `m_total` is
std::nullopt.