[clang] [NFC][Clang][AST] Use `llvm::copy` instead of `memcpy` in StringLiteral (PR #145187)

[via cfe-commits]

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@@ -1853,18 +1852,11 @@ class StringLiteral final
 
 public:
   /// This is the "fully general" constructor that allows representation of
-  /// strings formed from multiple concatenated tokens.
+  /// strings formed from multiple concatenated tokens. This same constructor
+  /// also work for a single token case.
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Sirraide wrote:

```suggestion
  /// strings formed from one or more concatenated tokens.
```

https://github.com/llvm/llvm-project/pull/145187