Rollup merge of rust-lang#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 (rust-lang#33434, rust-lang#50604). Thanks to rust-lang#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>.)

Author: kennytm