Allow bytes-only mach-o corefile to load into lldb

[jasonmolenda]

Summary

ProcessMachCore::DoLoadCore has an error check that if it found no thread contexts (LC_THREADs), it is an improperly formed corefile and will be rejected. We have a group that is creating memory-only corefiles, and will provide threads from an operating system or scripted process python plugin that they write. This removes the error handling from DoLoadCore -- I can't remember this catching problems with corrupt corefiles in real world use.

I haven't cooked up a test for it at this point; it would be possible to run a user process, process save-core it, write a utility program that rewrites the corefiles to remove the LC_THREAD load commands, loads it into lldb, and confirms that memory can still be examined. But given the simplicity of the change, I don't feel super motivated to write all that. I'm open to it though, if other people feel it would be a good addition to our test coverage.

[mib]

LGTM!

lldb/source/Plugins/Process/mach-core/ProcessMachCore.cpp

Status ProcessMachCore::DoLoadCore() {
   }
 
   ObjectFile *core_objfile = m_core_module_sp->GetObjectFile();
   if (core_objfile == nullptr) {
     error.SetErrorString("invalid core object file");
     return error;
   }
 
-  if (core_objfile->GetNumThreadContexts() == 0) {
-    error.SetErrorString("core file doesn't contain any LC_THREAD load "
-                         "commands, or the LC_THREAD architecture is not "
-                         "supported in this lldb");
-    return error;
-  }
-
   SetCanJIT(false);
 
   // The corefile's architecture is our best starting point.
   ArchSpec arch(m_core_module_sp->GetArchitecture());
   if (arch.IsValid())
     GetTarget().SetArchitecture(arch);
 
   CreateMemoryRegions();