[lldb] Make sure Blocks always have a parent (#117683)

It's basically true already (except for a brief time during
construction). This patch makes sure the objects are constructed with a
valid parent and enforces this in the type system, which allows us to
get rid of some nullptr checks.

GitOrigin-RevId: e8a01e75e6b17e84710e7cc3bfb70d95a3a696a2
7 files changed