[clang] [NFC][Static Analyzer] Rename and discuss about `NotNullConstraint` & `NotNullBufferConstraint` (PR #131374)

[Ziqing Luo via cfe-commits]

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@@ -436,9 +436,9 @@ class StdLibraryFunctionsChecker
                                llvm::raw_ostream &Out) const override;
 
     ValueConstraintPtr negate() const override {
-      NotNullBufferConstraint Tmp(*this);
+      BufferNullnessConstraint Tmp(*this);
       Tmp.CannotBeNull = !this->CannotBeNull;
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ziqingluo-90 wrote:

The negation of the constraint does not fully match my expectation: this constraint is saying `size == 0 || buf != 0`. So the negation is suppose to be `size != 0 && buf == 0`. The implementation looks more like `size == 0 || buf == 0` to me.

In general, how is the negation of a constraint used? I imagine that sometimes the engine needs to split the path by assuming '_C_' and '!_C_' for a constraint '_C_'? If the negation can be used as an assumption for a path, we should be serious about its correctness. Additionally, `describe` and `describeArgumentValue` need to report differently depending on whether the constraint is negated or not.

https://github.com/llvm/llvm-project/pull/131374