[Patch] Make errors and crashes distinct even with LLVM_DISABLE_CRASH_REPORT

[Rafael Espíndola]

It looks like we went a bit too far with LLVM_DISABLE_CRASH_REPORT on
windows. Some failures, like an assert(0) not only disable the crash
reporting, they also make the program exit as if it was a regular
error.

This creates problems for enabling pipefail, as some tests would need
"not --crash" on unix and just "not" on windows.

The attached patch removes CRTReportHook. I don't claim to fully
understand this, but the behavior looks like what we want:

* check-all is clean
* Without  LLVM_DISABLE_CRASH_REPORT  clang -cc1 will open the crash
dialog when it crashes (#pragma clang __debug crash).
* With  LLVM_DISABLE_CRASH_REPORT we will print a backtrace and no
crash report shows up.
* "not --crash" returns zero when it runs a crashing clang.

Cheers,
Rafael
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