Previously, the SEH codepath in CrashRecoveryContext didn't create a CrashRecoveryContextImpl. The other codepaths (VEH and Unix) were creating it.
When running with -fintegrated-cc1, this is needed to handle exit() as a jump to CrashRecoveryContext's exception filter, through a call to RaiseException. In that situation, we need a user-defined exception code, which is later interpreted as an exit() by the exception filter. Which in turn needs to set RetCode accordingly, inside the exception filter, and before calling HandleCrash().
This is a support patch for D73742.