[clang-tools-extra] [clang-tidy][readability-identifier-length] Add a line count threshold (PR #185319)

[Alex Dutka via cfe-commits]

dutkalex wrote:

> Would you please add a ReleaseNotes entry?

@zeyi2 This is done. For some reason the CI is not running the tests on the latest commit, but the PR is otherwise ready for review (unless if there are other oversights I should address first obviously).

https://github.com/llvm/llvm-project/pull/185319