[clang] [dataflow] Parse formulas from text (PR #66424)

[via cfe-commits]

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@@ -87,6 +87,9 @@ class alignas(const Formula *) Formula {
                          ArrayRef<const Formula *> Operands,
                          unsigned Value = 0);
 
+  // Parse Formulas using Arena rather than caling this function directly.
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martinboehme wrote:

```suggestion
  // Don't call this function directly. Use `Arena::parseFormula()` instead.
```

Nit: When I began reading the comment, "Parse Formulas using Arena" initially sounded like a description of what the function is doing. "Don't call directly" seems like the most helpful thing to put at the beginning.

https://github.com/llvm/llvm-project/pull/66424