[Support] When using SEH, create a impl instance for CrashRecoveryContext. NFCI.

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. This in turn needs to set RetCode accordingly, *inside* the exception filter, and *before* calling HandleCrash().

Differential Revision: https://reviews.llvm.org/D74078