[clang-tools-extra] [llvm] [clang-tidy] New check `misc-header-guard` (PR #177315)

[Yanzuo Liu via cfe-commits]

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zwuis wrote:

We are migrating `.rst` to `.md`. Actually there are `.md` files in this directory in this branch :)

https://github.com/llvm/llvm-project/pull/177315