gdb: Print allocated object's type name instead of full LSA migrator
gdb: Print allocated object's type name instead of full LSA migrator

Before:

  0x6000302604e0: live {_vptr.migrate_fn_type = 0x3797a00 <vtable for standard_migrator<cache_entry>+16>, _migrators = std::any containing seastar::lw_shared_ptr<(anonymous namespace)::migrators> = {[contained value] = {_p = 0x600000080a80}}, _align = 8, _index = 0} @ 0x6000302604e8

After:

  0x6000302604e0: live cache_entry @ 0x6000302604e8

scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla