[lldb] Make LLDB resilient against failing dyld introspection SPIs

[lldb] Make LLDB resilient against failing dyld introspection SPIs

Make LLDB resilient against failing dyld introspection SPIs:

 - dyld_process_create_for_current_task
 - dyld_process_snapshot_create_for_process
 - dyld_process_snapshot_get_shared_cache

These can all fail and return a nullptr. Instead of having an assert,
which doesn't really make sense, as we have no control over whether
these calls succeed or not, bail out gracefully and use the fallback
logic.

rdar://98070414

Differential revision: https://reviews.llvm.org/D131110