windows stack traces unable to handle debug info from stage2 compiler

[andrewrk]

If you get a stack trace from stage2, (e.g. a panic in the self hosted compiler), first of all it takes a long time, and secondly it prints error.MissingDebugInfo rather than a stack trace.

[mlugg]

This has probably been fixed for a very long time, and we have good test coverage for it as of b64535e.