Use _Unwind_GetRegionStart() if _Unwind_GetIP() is not available

[krlmlr]

Follow-up to #43. This gives correct function name, but still incorrect file+line information (and thus failing tests) on mingw64.

[ianlancetaylor]

Thanks, but I would rather not use a workaround like this. As you say, it gives incorrect results, and incorrect results can be much more confusing than missing results.

More generally, being able to backtrace with the correct PC value is essential for C++ exceptions. Being unable to get the correct PC makes some kinds of C++ programming essentially impossible. So this seems like a fatal bug that has to be fixed for the system to be at all usable. Since it has to be fixed, there isn't much point to working around the case where it is not fixed.

[krlmlr]

Thanks for the heads up. I agree with the sentiment.

For mingw64, only the _Unwind_Backtrace() code path is broken. Exception handling seems to work correctly.

The results this PR gives are good enough to get practical value out of the backtrace, only the file+line information is off. This may be sufficient for some use cases.

[ianlancetaylor]

Thanks for the info. I can only repeat that returning the file/line information gives incorrect results, and in my opinion for something like libbacktrace incorrect results are generally worse than no results.

A change like this would be fine with me if we somehow arranged to only return the function name, not the file/line information.