[clang] [analyzer] Ignoring `T v=v; ` idiom for uninitialized variable checker and dead store checker (PR #187530)

[Ella Ma via cfe-commits]

================
@@ -431,6 +431,12 @@ class DeadStoreObs : public LiveVariables::Observer {
                   //  bug.
                   if (isa<ParmVarDecl>(VD) && VD->getType()->isScalarType())
                     return;
+                  // Special case: check for self-initializations.
+                  //
+                  //  e.g. int x = x;
+                  //
+                  if (VD == V)
----------------
Snape3058 wrote:

This is not checked by the deadcode.DeadStores checker.

```
void DeadStoreObs::CheckVarDecl(const VarDecl *VD, ...) ...
```

For the uninitialized variable checker, this actually leads to a false negative now.

```
struct S { int x = x; };
int foo() {
  S s;
  return s.x;
}
```

The uninitialized read to `s.x` is not reported. But I think that this FN needs to be considered in another PR.

Besides, this case is currently warned by the frontend warnings.

https://github.com/llvm/llvm-project/pull/187530