[clang-tools-extra] [clang-tidy] add new check readability-enum-initial-value (PR #86129)

[Piotr Zegar via cfe-commits]

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@@ -0,0 +1,75 @@
+.. title:: clang-tidy - readability-enum-initial-value
+
+readability-enum-initial-value
+==============================
+
+Detects explicit initialization of a part of enumerators in an enumeration, and
+relying on compiler to initialize the others.
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PiotrZSL wrote:

Maybe:
```
Enforces consistent style for enumerators' initialization, covering three styles: none, first only, or all initialized explicitly.
```

https://github.com/llvm/llvm-project/pull/86129