[clang-tools-extra] [clang-tidy] Add performance-substr-self-assignment check (PR #209657)

[Zeyi Xu via cfe-commits]

zeyi2 wrote:

> A note on the `add_new_check.py` change included in the last commit: converting the check documentation to Markdown broke `test/clang-tidy/infrastructure/update-checks-list.test`, because `add_new_check.py --update-docs` only scans `*.rst` files when regenerating `docs/clang-tidy/checks/list.rst`. A check whose documentation is a `.md` file silently drops out of the regenerated list, which the test then reports as `list.rst` being out of date.

Thanks, FYI there is already another PR that tries to address the same problem here: https://github.com/llvm/llvm-project/pull/210574



https://github.com/llvm/llvm-project/pull/209657