[libc-commits] [libc] [libc][clang-tidy] Add llvm-header-guard to get consistant naming and prevent file copy/paste issues. (PR #66477)

[Guillaume Chatelet via libc-commits]

gchatelet wrote:

> What should we do for directories starting with two underscores?

It seems like `libcxx` is [fine with multiple underscores](https://github.com/llvm/llvm-project/blob/d861b3183cb0baff81120874a16f22e9a4bc9e60/libcxx/include/__algorithm/all_of.h).

https://github.com/llvm/llvm-project/pull/66477