`this == nullptr` results in undefined behavior

[ShortDevelopment]

Issue

‘this’ pointer cannot be null in well-defined C++ code; pointer may be assumed to always convert to true

Tasks

• [ ] Add compiler warning clang: -Wundefined-bool-conversion msvc: ??
• [x] #6847
• [ ] IR.cpp
• [x] Find further places where this may happen

[ppenzin]

This warning should now be enabled in Clang by default, though didn't catch this earlier due to -Werror, I suspect it wasn't enabled in the version from the package manager.

[ShortDevelopment]

https://github.com/chakra-core/ChakraCore/blob/2af598f04ab508f9231d6e26f0f82f5a57561413/lib/Backend/IR.cpp#L4852

[rhuanjl]

I had a look IR.cpp and the invalid check is being done part of a Debug only feature; so it's not as critical a concern as ImmutableList.

I've also done a sequence of searches in the codebase and I don't think there's anything else.