[Frontend] Add support for implicit import of _Backtracing

Once the API has gone through Swift Evolution, we will want to implicitly
import the _Backtracing module.  Add code to do that, but set it to off
by default for now.

rdar://105394140

lib/DriverTool/autolink_extract_main.cpp

@@ -250,7 +250,8 @@ int autolink_extract_main(ArrayRef<const char *> Args, const char *Argv0,
       {"-lswiftCore", false},
       {"-lswift_Concurrency", false},
       {"-lswift_StringProcessing", false},
-      {"-lswift_RegexParser", false}
+      {"-lswift_RegexParser", false},
+      {"-lswift_Backtracing", false},
   };
 
   // Extract the linker flags from the objects.