[C++] Improper escaping of table named "export"

[jameskuszmaul-brt]

If you create a table named export the generated code is inconsistent and ends up with the errors in https://github.com/google/flatbuffers/runs/5382749465?check_suite_focus=true

In file included from /home/runner/work/flatbuffers/flatbuffers/tests/test.cpp:24:
/home/runner/work/flatbuffers/flatbuffers/tests/monster_test_generated.h:1231:12: error: use of undeclared identifier 'export_TypeTable'; did you mean 'exportTypeTable'?
    return export_TypeTable();
           ^~~~~~~~~~~~~~~~
           exportTypeTable
/home/runner/work/flatbuffers/flatbuffers/tests/monster_test_generated.h:117:38: note: 'exportTypeTable' declared here
inline const flatbuffers::TypeTable *exportTypeTable();
                                     ^
[ [98](https://github.com/google/flatbuffers/runs/5382749465?check_suite_focus=true#step:4:98)%] Building CXX object CMakeFiles/flattests.dir/src/code_generators.cpp.o
In file included from /home/runner/work/flatbuffers/flatbuffers/tests/test_builder.cpp:1:
In file included from /home/runner/work/flatbuffers/flatbuffers/tests/test_builder.h:8:
/home/runner/work/flatbuffers/flatbuffers/tests/monster_test_generated.h:1231:12: error: use of undeclared identifier 'export_TypeTable'; did you mean 'exportTypeTable'?
    return export_TypeTable();
           ^~~~~~~~~~~~~~~~
           exportTypeTable
/home/runner/work/flatbuffers/flatbuffers/tests/monster_test_generated.h:117:38: note: 'exportTypeTable' declared here
inline const flatbuffers::TypeTable *exportTypeTable();
                                     ^
1 error generated.

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