[analyzer] Fix crashing getSValFromInitListExpr for nested initlists

In the following example, we will end up hitting the `llvm_unreachable()`:
https://godbolt.org/z/5sccc95Ec
```lang=C++
enum class E {};
const E glob[] = {{}};
void initlistWithinInitlist() {
  clang_analyzer_dump(glob[0]); // crashes at loading from `glob[0]`
}
```

We should just return `std::nullopt` instead for these cases.
It's better than crashing.

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D146538

(cherry picked from commit 558b46fd)