[lldb] Refactor ObjC/NSException.cpp (cleanup, avoid code duplication). NFC.

- Refactor reading of NSException fields into ExtractFields method to avoid code duplication.
- Remove "m_child_ptr" field, as it's not used anywhere.
- Clang-format.

Differential Revision: https://reviews.llvm.org/D44073

llvm-svn: 346679