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?