[Basic] Fallback to dumping to stderr when backtracing is disabled

Make sure we still output the error message when pretty backtracing
is disabled.

lib/Basic/Assertions.cpp

@@ -120,6 +120,10 @@ static void _abortWithMessage(llvm::StringRef message) {
   // crash reporter.
   PrettyStackTraceMultilineString trace(message);
 
+  // If pretty backtracing is disabled, fall back to dumping to stderr.
+  if (!llvm::SavePrettyStackState())
+    llvm::errs() << message << '\n';
+
   abort();
 }