Add a new SBProcess:: GetCoreFile() API

Summary:
This diff added a new `SBProcess::GetCoreFile()` to retrieve target dump file spec used for dump debugging. Note: this is different from the main executable module spec

Facebook:
This is required for `auto-load-debuginfo` Python script to locate METADATA file next to coredump file.

To achieve this, the patch hoists `m_core_file` into `PostMortemProcess` for sharing.

Test Plan:
A new unit test is added.
Tested end-to-end in `auto-load-debuginfo` Python script.

Reviewers: wanyi, hyubo, #lldb_team

Reviewed By: wanyi

Subscribers: #lldb_team

Differential Revision: https://phabricator.intern.facebook.com/D45255862

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

@@ -404,7 +404,7 @@ bool ProcessMachCore::LoadBinariesViaMetadata() {
 
   // LoadCoreFileImges may have set the dynamic loader, e.g. in
   // PlatformDarwinKernel::LoadPlatformBinaryAndSetup().
-  // If we now have a dynamic loader, save its name so we don't 
+  // If we now have a dynamic loader, save its name so we don't
   // un-set it later.
   if (m_dyld_up)
     m_dyld_plugin_name = GetDynamicLoader()->GetPluginName();

lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py

@@ -838,7 +838,8 @@ def test_get_core_file_api(self):
         Test SBProcess::GetCoreFile() API can successfully get the core file.
         """
         core_file_name = "linux-x86_64.core"
-        target = self.dbg.CreateTarget("linux-x86_64.out")
+        self.runCmd("settings set use-module-list-dyld true")
+        target = self.dbg.CreateTarget(None)
         process = target.LoadCore(core_file_name)
         self.assertTrue(process, PROCESS_IS_VALID)
         self.assertEqual(process.GetCoreFile().GetFilename(), core_file_name)