The Win32 unwinder has to locate function entry records for the
`StackWalk64()` API to use; the code that was doing this was
incorrect and sometimes found the wrong record, which would result
in a corrupted stack walk.
rdar://172911008
On-crash backtracing is basically there for 64-bit Windows. It
won't work on 32-bit because of a Swift compiler issue, and there
is a little more work to do yet, but it is now working!
rdar://101623384
This doesn't have a working symbolicator yet, but it does build and
it can obtain a basic backtrace.
It also doesn't include a working `swift-backtrace` program yet.
rdar://101623384
Alastair Houghton