fix lookup test failure

Summary:
Related to D79487113.

The changes to the `GetChildAtIndex` API from llvm/llvm-project#140065 also seems to affect other HPHP-lldb helper functions.

In the test, we used to see something like
```
(HPHP::Func *) m_s = NULL
```
But now, it's like
```
(HPHP::Func *) [0] = NULL
```

See the [Sandcastle test failure](https://www.internalfb.com/sandcastle/workflow/725079540015841236/artifact/actionlog.725079540171251401.stderr.1?selectedLines=650-650-117-117).

# Additional context

The test failure is in `test_lookup.py`, specifically the `LookupCommandTestCase.test_lookup_func_command`.

The failure happens in the `lookup func 0` command. It appears to be because we're now trying to read from some `AtomicLowPtrVector` as an array, which now causes the `func_ptr` to be displayed. The trace roughly starts from [lookup.py (_lookup_func_prep)](https://www.internalfb.com/code/fbsource/[bdc39ed90991]/fbcode/hphp/tools/lldb/lookup.py?lines=226). The change in behavior eventually comes from the `idx.atomic_low_ptr_vector_at` call around [here](https://www.internalfb.com/code/fbsource/[506e8e599f3c]/fbcode/hphp/tools/lldb/lookup.py?lines=58).

It seems this is because we have a call within `idx.atomic_low_ptr_vector_at` (eventually) that calls `GetChildAtIndex(0, .., True)`, where `True` is the `treat_as_array` param.

To look into the issue, I reproed using this script P1896003453.
```
lldb $HHVM -o "command script import /data/users/toyang/fbsource/fbcode/hphp/tools/lldb/hhvm.py" -o "b lookupObjMethod" -o "r" --source "$SCRIPT" -- --file /data/users/toyang/fbsource/fbcode/hphp/test/quick/method2.php -vEval.Jit=0
```

mdko is looking into if this new behavior is desirable.

Reviewed By: mdko

Differential Revision: D79827088

fbshipit-source-id: 0c5eaf91bcc148e3c9347bfe7b59d1f03e7a45fd

Author: Tom Yang

hphp/tools/lldb/test/test_lookup.py

@@ -17,7 +17,7 @@ def test_lookup_func_command(self) -> None:
         self.run_until_breakpoint("lookupObjMethod")
         _, output = self.run_commands(["lookup func 0"])
         self.assertRegex(
-            output.strip(), r"\(HPHP::Func \*\) (m_s|\$\d+) = (NULL|nullptr)"
+            output.strip(), r"\(HPHP::Func \*\) (m_s|\$\d+|\[\d+\]) = (NULL|nullptr)"
         )