[analyzer] Simplify SVal for simple NonLoc->Loc casts (#66463)

NonLoc symbolic SVal to Loc casts are not supported except for
nonloc::ConcreteInt.

This change simplifies the source SVals so that the more casts can go
through nonloc::ConcreteInt->loc::ConcreteInt path. For example:

```c
  void test_simplified_before_cast_add(long long t1) {
    long long t2 = t1 + 3;
    if (!t2) {
      int *p = (int *) t2;
      clang_analyzer_eval(p == 0); // expected-warning{{TRUE}}
    }
  }
```
If simplified, `t2` is 0, resulting `p` is nullptr, otherwise `p` is
unknown.

Fixes #62232