[clang] [NFC][LifetimeSafety]: Track assignment history within a single CFGBlock (PR #196075)

[Yuan Suo via cfe-commits]

================
@@ -0,0 +1,39 @@
+//===- AssignmentQuery.cpp - C++ Lifetime Safety Checker --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the LifetimeChecker, which detects use-after-free
+// errors by checking if live origins hold loans that have expired.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_ANALYSIS_ANALYSES_ASSIGNMENTQUERY_H
+#define LLVM_CLANG_ANALYSIS_ANALYSES_ASSIGNMENTQUERY_H
+
+#include "clang/Analysis/Analyses/LifetimeSafety/Facts.h"
+#include "clang/Analysis/Analyses/LifetimeSafety/LoanPropagation.h"
+
+namespace clang::lifetimes::internal {
+using AssignmentPair = std::pair<DestOriginEntity, SrcOriginEntity>;
----------------
suoyuan666 wrote:

Do you mean maintaining LHS and RHS as a single flat list because the LHS of one expression often becomes the RHS of the next?

I considered that, but it seems insufficient for retrieving the correct `LHS->ExprLoc` mapping when generating `Sema` diagnostics later.

Take the following case as an example:

```c
int *s;
{
  int tgt = 2;
  int *a = &tgt;
  int *e = a;
  s = e;
}
(void)s;
```

During the reverse search, we can identify the `DeclRefExpr` for `e` in `s = e`.

However, we cannot ignore the `ValueDecl` corresponding to `int *a`. If we don't track them as explicit pairs, it becomes difficult to precisely pinpoint and report the `ExprLoc` of `e` in the subsequent diagnostic notes.

https://github.com/llvm/llvm-project/pull/196075