Filter non-external static members from SBType::GetFieldAtIndex.

See [[ https://github.com/llvm/llvm-project/issues/55040 | issue 55040 ]] where static members of classes declared in the anonymous namespace are incorrectly returned as member fields from lldb::SBType::GetFieldAtIndex(). It appears that attrs.member_byte_offset contains a sentinel value for members that don't have a DW_AT_data_member_location.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D124409