[flang][nfc] Tidy up in CodeGen.cpp and TypeConverter.h

Some header `#include`s that belong in TypeConverter.h rather than in
CodeGen.cpp are moved accordingly. As TypeConverter.h is currently only
used in CodeGen.cpp, this wouldn't cause any build issues (ultimately,
all the required headers are included). However, this would become a problem
when including TypeConverter.h elsewhere.

Also, from [1], "(...) include all of the header files that you are
using ". This is currently not the case in TypeConverter.h and hence the
`#include`s should be moved.

I've also added missing namespace qualifiers in a few places in
TypeConverter.h

[1] https://llvm.org/docs/CodingStandards.html#include-as-little-as-possible

Differential Revision: https://reviews.llvm.org/D113569