[llvm] [YAMLParser] Improve plain scalar spec compliance (PR #68946)

[Walter Erquinigo via llvm-commits]

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@@ -392,6 +392,10 @@ class Scanner {
   ///        Pos is whitespace or a new line
   bool isBlankOrBreak(StringRef::iterator Position);
 
+  /// Return true if the minimal well-formed code unit subsequence at
+  ///        Pos is considered a "safe" character for plain scalars.
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walter-erquinigo wrote:

```suggestion
  /// Position is considered a "safe" character for plain scalars.
```

https://github.com/llvm/llvm-project/pull/68946