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

[via cfe-commits]

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@@ -137,5 +140,46 @@ TEST_F(ArenaTest, Interning) {
   EXPECT_EQ(&B1.formula(), &F1);
 }
 
+TEST_F(ArenaTest, Parse) {
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martinboehme wrote:

```suggestion
TEST_F(ArenaTest, ParseFormula) {
```

Matches name of the function, and we might conceivably add other parsing functions to `Arena` in the future.

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