[clang-tidy] Fix segfault in cppcore-guidelines-special-member-functions check

Summary:
Use a set rather than a vector of defined special member functions so
that multiple declarations of the same function are only counted once.

Move some private static member functions into the cpp file.

Run clang-format on header.

Reviewers: ericLemanissier, Prazek, aaron.ballman

Subscribers: Prazek, cfe-commits, nemanjai

Projects: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D23008

llvm-svn: 277523