This allows the caller of the function to add a note about how to fix the stack exhaustion. This can be useful to aid the user. For example it can be used to show a help message for https://bugs.llvm.org/show_bug.cgi?id=14030
I sort of feel like we don't want to go this route because we want to avoid calling runWithSufficientStackSpace whenever possible, but I am not strongly opposed to this patch. It should be combined with the usage of the new functionality, however, so that it gets adequate test coverage.