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

[Yanzuo Liu via cfe-commits]

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+.. title:: clang-tidy - llvm-header-guard
+
+misc-header-guard
+=================
+
+Finds and fixes header guards.
+
+Options
+-------
+
+.. option:: HeaderDirs
+
+   A list of one or more header directory names. The list is searched for the
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zwuis wrote:

A semicolon-separated list

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Please document that this option only treat `/` as path separator.

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It seems that we support this form `HeaderDirs: include/subdir`. It would be nice to document it.

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