Improve our error handling on windows.

* Remove LLVM_ENABLE_CRT_REPORT. LLVM_DISABLE_CRASH_REPORT made it redundant.
* set Return to 1, so that we get a stack trace on failure.
* don't call _exit, so that we get a negative exit value and "not --crash"
  correctly differentiates crashes and regular errors.

This is a bit experimental since the documentation on this interface is sparse.
It doesn't bring up a dialog on my windows setup, but feel free to revert
if it causes problem for your setup (and let me know what it is so that I
can try to fix this patch).