Remove _cpp_ auto-escaping

[bernardnormier]

C++ (unlike C#) does not provide a standard mechanism to escape identifiers that conflict with keywords.

slice2cpp auto-escapes identifiers that conflict with C++ keywords by adding a _cpp_ prefix to the mapped C++ identifier:

https://doc.zeroc.com/ice/3.7/language-mappings/c++11-mapping/client-side-slice-to-c++11-mapping/c++11-mapping-for-identifiers

The resulting identifier (say _cpp_while) is not particularly nice.

Proposal

Update slice2cpp to not auto-escape identifiers that conflict with C++ keywords. When this happens, the generated C++ code won't compile and users should use cpp:identifier:xxx to map this Slice identifier to the C++ identifier of their choice.

See #2864.