[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

clang/lib/StaticAnalyzer/Core/RegionStore.cpp

@@ -1849,8 +1849,12 @@ std::optional<SVal> RegionStoreManager::getSValFromInitListExpr(
     // Go to the nested initializer list.
     ILE = IL;
   }
-  llvm_unreachable(
-      "Unhandled InitListExpr sub-expressions or invalid offsets.");
+
+  assert(ILE);
+
+  // FIXME: Unhandeled InitListExpr sub-expression, possibly constructing an
+  //        enum?
+  return std::nullopt;
 }
 
 /// Returns an SVal, if possible, for the specified position in a string

clang/test/Analysis/initialization.cpp

@@ -249,3 +249,10 @@ void glob_array_parentheses1() {
   clang_analyzer_eval(glob_arr9[1][2] == 7); // expected-warning{{TRUE}}
   clang_analyzer_eval(glob_arr9[1][3] == 0); // expected-warning{{TRUE}}
 }
+
+enum class E {};
+const E glob[] = {{}};
+void initlistWithinInitlist() {
+  // no-crash
+  clang_analyzer_dump(glob[0]); // expected-warning-re {{reg_${{[0-9]+}}<enum E Element{glob,0 S64b,enum E}>}}
+}