Update use of GetChildAtIndex to reflect changes in API for next version of LLDB

Summary:
The API for GetChildAtIndex was changed in llvm/llvm-project#140065. So that "True" at the end means something different now ("treat_as_array" rather than "can_create_synthetic"). This change will be present in the next version of LLDB used internally.

Happily it looks like there's a way to update things that both doesn't break the tests with the current version while updating it for the next version; otherwise we'd have to use something like [this](
https://www.internalfb.com/code/fbsource/[959f6221198f]/fbcode/sand/tests/lldb_test_case.py?lines=53) in order to gate things (leaving this as a note for future changes).

Reviewed By: zhyty

Differential Revision: D79487113

fbshipit-source-id: 946f0f6d8fee72cfaf85c710fb613dda89003c6f

Author: mdko

hphp/tools/lldb/frame.py

@@ -338,7 +338,7 @@ def php_line_number_from_repo(func: lldb.SBValue, pc: int) -> typing.Optional[in
     line_table_size = sizeof.sizeof(line_table)
     assert line_table_size is not None
     for i in range(line_table_size):
-        line_entry = idx.at(line_table, i)
+        line_entry = idx.at(line_table, i, False)
 
         if line_entry is None:
             break

hphp/tools/lldb/idx.py

@@ -20,13 +20,14 @@
 def at(
     ptr: lldb.SBValue,
     idx: typing.Union[int, lldb.SBValue],
+    treat_as_array: bool = True,
 ) -> typing.Optional[lldb.SBValue]:
     """Access ptr[idx]"""
 
     if isinstance(idx, lldb.SBValue):
         idx = idx.unsigned
 
-    val = ptr.GetChildAtIndex(idx, lldb.eDynamicDontRunTarget, True)
+    val = ptr.GetChildAtIndex(idx, lldb.eDynamicDontRunTarget, treat_as_array)
 
     utils.debug_print(
         f"idx.at(ptr={ptr} ({ptr.type.name}), idx={idx}) = {val.unsigned} (0x{val.unsigned:x}) (error={val.GetError()})"