[clang-tidy] Handle member variables in readability-simplify-boolean-expr

- Add readability-simplify-boolean-expr test cases for member variables
Fixes PR40179

Patch by LegalizeAdulthood.

Reviewers: alexfh, hokein, aaron.ballman, JonasToth

Reviewed By: JonasToth

Subscribers: jdoerfert, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D56323

llvm-svn: 360882