THRIFT-5637: Add `no_constructors` option to C++ compiler

[alkis]

This PR adds the option no_constructors in the C++ generator. This option elides declaring and defining:

• constructor/destructor
• move/copy constructor/assignment

In addition this PR fixes some minor issues:

1. gen_no_skeleton_ documentation was incorrect
2. when templates are requested, avoid any virtual functions

Allowing the compiler to emit the default set of constructor/destructor/assignment operators makes the code faster as it allows inlining when they are trivial. This improves creating deep/elaborate thrift objects.

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[alkis]

Perhaps the option should be aggregates instead of no_constructor and classes be made into structs instead?

[alkis]

@Jens-G could you advise how to move this forward?

[Jens-G]

@Jens-G could you advise how to move this forward?

The usual way would be that someone fom the C++ folks looks at it and decides how to continue.

[alkis]

Awesome, thank you @Jens-G!