Rollup merge of #50644 - kennytm:read-appveyor-dump, r=alexcrichton

AppVeyor: Read back trace from crash dump on failure.

This is an attempt to debug spurious access violations on Windows (#33434, #50604). Thanks to #50276, there should be minidumps to read when rustc segfault.

(The implementation is based on <https://github.com/springmeyer/travis-coredump/blob/master/test.bat>.)