Poor escaping of keywords in C++ Union codegen

[jkuszmaul]

I seem to be able to generate C++ code from schemas containing unions that does not compile if the unions contain C++ keywords in the correct spots. See #8352 as a demonstration which makes use of the existing keyword_test.fbs to generate invalid C++.

May get to this myself, may not. Creating this issue to document the presence of some remaining work to be done around keyword escaping in C++.