[clang-tidy] initial version of readability-convert-member-functions-to-static

Summary:
Finds non-static member functions that can be made ``static``.

I have run this check (repeatedly) over llvm-project. It made 1708 member functions
``static``. Out of those, I had to exclude 22 via ``NOLINT`` because their address
was taken and stored in a variable of pointer-to-member type (e.g. passed to
llvm::StringSwitch).
It also made 243 member functions ``const``. (This is currently very conservative
to have no false-positives and can hopefully be extended in the future.)

You can find the results here: https://github.com/mgehre/llvm-project/commits/static_const_eval

Reviewers: alexfh, aaron.ballman

Subscribers: mgorny, xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61749

llvm-svn: 366265