Make generated file use valid C++ identifiers

[mgeplf]

• according to https://en.cppreference.com/w/cpp/language/identifiers

• identifiers with a double underscore anywhere;
• identifiers that begin with an underscore followed by an uppercase letter;
• in the global namespace, identifiers that begin with an underscore.

• clang w/ -Weverything is including -Wreserved-id-macro, which fails with the current macro naming scheme.

• cleaned up tests a bit

[mgeplf]

Test appear to be failing w/ clang>=15.0.7, but work with clang<=14.0.6.

[mgeplf]

Test appear to be failing w/ clang>=15.0.7, but work with clang<=14.0.6.

I tried locally, and with llvm-16 then the python clang==16.0.1.1works, so to not have to pinclang`, updated library versions would need to be installed.