[clang-tidy] Non-private member variables in classes (MISRA, CppCoreGuidelines, HICPP)

Summary:
Finds classes that not only contain the data (non-static member variables),
but also have logic (non-static member functions), and diagnoses all member
variables that have any other scope other than `private`. They should be
made `private`, and manipulated exclusively via the member functions.

Optionally, classes with all member variables being `public` could be
ignored, and optionally all `public` member variables could be ignored.

Options
-------

* IgnoreClassesWithAllMemberVariablesBeingPublic

  Allows to completely ignore classes if **all** the member variables in that
  class have `public` visibility.

* IgnorePublicMemberVariables

  Allows to ignore (not diagnose) **all** the member variables with `public`
  visibility scope.

References:
* MISRA 11-0-1 Member data in non-POD class types shall be private.
* https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c2-use-class-if-the-class-has-an-invariant-use-struct-if-the-data-members-can-vary-independently
* https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rc-private
* https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rh-protected

Reviewers: JonasToth, aaron.ballman, alexfh, hokein, xazax.hun

Reviewed By: aaron.ballman

Subscribers: Eugene.Zelenko, zinovy.nis, cfe-commits, rnkovacs, nemanjai, mgorny, xazax.hun, kbarton

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D52771

llvm-svn: 344757