[Support] Clarify CrashRecoveryContext exception codes on Windows

[aganea]

Summary

Explain why these particular exception codes are used.
As suggested in https://reviews.llvm.org/D142224#4113763

[mstorsjo]

LGTM, thanks! (Maybe add an NFC tag to the commit subject?)

llvm/lib/Support/CrashRecoveryContext.cpp

bool CrashRecoveryContext::RunSafely(function_ref<void()> Fn) {
   Fn();
   return true;
 }
 
 #endif // !_MSC_VER
 
 [[noreturn]] void CrashRecoveryContext::HandleExit(int RetCode) {
 #if defined(_WIN32)
-  // SEH and VEH
+  // Since the exception code is actually of NTSTATUS type, we use the
+  // Microsoft-recommended 0xE prefix, to signify that this is a user error.
+  // This value is a combination of the customer field (bit 29) and severity
+  // field (bits 30-31) in the NTSTATUS specification.
   ::RaiseException(0xE0000000 | RetCode, 0, 0, NULL);
 #else
   // On Unix we don't need to raise an exception, we go directly to
   // HandleCrash(), then longjmp will unwind the stack for us.
   CrashRecoveryContextImpl *CRCI = (CrashRecoveryContextImpl *)Impl;
   assert(CRCI && "Crash recovery context never initialized!");
   CRCI->HandleCrash(RetCode, 0 /*no sig num*/);
 #endif
   llvm_unreachable("Most likely setjmp wasn't called!");
 }
 
 bool CrashRecoveryContext::isCrash(int RetCode) {
 #if defined(_WIN32)
-  // On Windows, the high bits are reserved for kernel return codes. Values
-  // starting with 0x80000000 are reserved for "warnings"; values of 0xC0000000
-  // and up are for "errors". In practice, both are interpreted as a
-  // non-continuable signal.
+  // On Windows, the code is interpreted as NTSTATUS. The two high bits
+  // represent the severity. Values starting with 0x80000000 are reserved for
+  // "warnings"; values of 0xC0000000 and up are for "errors". In practice, both
+  // are interpreted as a non-continuable signal.
   unsigned Code = ((unsigned)RetCode & 0xF0000000) >> 28;
   if (Code != 0xC && Code != 8)
     return false;
 #else
   // On Unix, signals are represented by return codes of 128 or higher.
   // Exit code 128 is a reserved value and should not be raised as a signal.
   if (RetCode <= 128)
     return false;