[Clang] Remove unused #pragma clang __debug handle_crash

As discussed in D70568, remove this because it isn't used anywhere, and I think it's better to go through real crashes for testing (#pragma clang __debug crash).
Also remove the support function llvm::CrashRecoveryContext::HandleCrash() which was added at the same time by @ddunbar.

Differential Revision: https://reviews.llvm.org/D74063