[lldb] Fix uninitialized memory access. (#150544)
lldb/test/API/functionalities/process_save_core_minidump/TestProcessSaveCoreMinidump64b.py
fails under msan with uninitialized memory access errors. The problem is
that a few structs are written to the dump without having been fully
initialized. This change makes them default-initialized so dumping the
fields that aren't explicitly written to won't trigger UB.
