[clang-tools-extra] [clang-tidy][NFC] Enable 'readability-simplify-boolean-expr' check (PR #158706)

[Baranov Victor via cfe-commits]

vbvictor wrote:

> Ok, should we merge some of the transformations or do we just close this PR?

If you have time and will, you can pick some transformations that you think benefit readability, and we can merge them after review. Most obvious is converting `== false` to `!`.

Wish we had more fine-tuning to `simplify-boolean-expr` check to enable only a part of it.

https://github.com/llvm/llvm-project/pull/158706