These missing header files wouldn't cause build failures as TypeConverter.h
is currently only included in CodeGen.cpp and that file includes everything
that's needed in TypeConverter.h. However, as per [1], "(...)
include all of the header files that you are using ".
I've also added missing namespace qualifiers in 2 places.
[1] https://llvm.org/docs/CodingStandards.html#include-as-little-as-possible
I don't think this one is needed.