devel/cdecl: upgrade to 6.6.

Release Notes:
Defaulted & deleted support
Defaulted & deleted special member functions are now supported.

Explain-by-default option
Both a command-line --explain/-e and explain-by-default set option have been added that assume a command that doesn't start with any command means explain.

Set option abbreviations
Unambiguous set option name abbreviations may now be used.

Virtual constexpr
Virtual constexpr declarations are now only correctly allowed in C++20.