[clang] [clang-format] Add .clang-format.ignore for ignoring files (PR #76327)

[Owen Pan via cfe-commits]

================
@@ -131,6 +131,24 @@ An easy way to create the ``.clang-format`` file is:
 
 Available style options are described in :doc:`ClangFormatStyleOptions`.
 
+You can create ``.clang-format-ignore`` files to make ``clang-format`` ignore
+certain files. A ``.clang-format-ignore`` file consists of patterns of file path
+names. It has the following format:
+- A blank line is skipped.
+- Leading and trailing spaces of a line are trimmed.
+- A line starting with a hash (``#``) is a comment.
+- A non-comment line is a single pattern.
+- The slash (``/``) is used as the directory separator.
+- A pattern is relative to the directory of the ``.clang-format-ignore`` file
+  (or the root directory if the pattern starts with a slash).
+- Patterns follow the rules specified in POSIX 2.13.1, 2.13.2, and Rule 1 of
+  2.13.3.
----------------
owenca wrote:

```suggestion
(or the root directory if the pattern starts with a slash).
- Patterns follow the rules specified in POSIX 2.13.1, 2.13.2, and Rule 1 of
2.13.3.
```

https://github.com/llvm/llvm-project/pull/76327