[lldb] Refactor away UB in SBValue::GetLoadAddress (#141799)

The problem was in calling GetLoadAddress on a value in the error state,
where `ValueObject::GetLoadAddress` could end up accessing the
uninitialized "address type" by-ref return value from `GetAddressOf`.
This probably happened because each function expected the other to
initialize it.

We can guarantee initialization by turning this into a proper return
value.

I've added a test, but it only (reliably) crashes if lldb is built with
ubsan.

lldb/test/API/python_api/value/TestValueAPI.py

@@ -269,7 +269,14 @@ class ValueAPITestCase(TestBase):
             frame0.FindVariable("another_fixed_int_ptr").GetValue(),
             "0xaa",
         )
+        a_null_int_ptr = frame0.FindVariable("a_null_int_ptr")
+        self.assertEqual(a_null_int_ptr.GetValue(), "0x0")
+
+        # Check that dereferencing a null pointer produces reasonable results
+        # (does not crash).
         self.assertEqual(
-            frame0.FindVariable("a_null_int_ptr").GetValue(),
-            "0x0",
+            a_null_int_ptr.Dereference().GetError().GetCString(), "parent is NULL"
+        )
+        self.assertEqual(
+            a_null_int_ptr.Dereference().GetLoadAddress(), lldb.LLDB_INVALID_ADDRESS
         )