[analyzer] Fix ValistChecker false-positive involving symbolic pointers

In the following example:

  int va_list_get_int(va_list *va) {
    return va_arg(*va, int); // FP
  }

The `*va` expression will be something like `Element{SymRegion{va}, 0, va_list}`.
We use `ElementRegions` for representing the result of the dereference.
In this case, the `IsSymbolic` was set to `false` in the
`getVAListAsRegion()`.

Hence, before checking if the memregion is a SymRegion, we should take
the base of that region.

Analogously to the previous example, one can craft other cases:

  struct MyVaList {
    va_list l;
  };
  int va_list_get_int(struct MyVaList va) {
    return va_arg(va.l, int); // FP
  }

But it would also work if the `va_list` would be in the base or derived
part of a class. `ObjCIvarRegions` are likely also susceptible.
I'm not explicitly demonstrating these cases.

PS: Check the `MemRegion::getBaseRegion()` definition.

Fixes #55009

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D124239