[LLDB SEV] Fix free after use of blocks destroyed when the module is replaced

Summary:
When auto-load debuginfo replaces a module, the SymbolVendor is also destroyed

```
      ObjectFile *obj_file = GetObjectFile();
      if (obj_file != nullptr) {
        LLDB_SCOPED_TIMER();
        m_symfile_up.reset(
            SymbolVendor::FindPlugin(shared_from_this(), feedback_strm));
        m_did_load_symfile = true;
        m_unwind_table.ModuleWasUpdated();
```

However, StackIDs can already have been created with a SymbolContext whose lifetime is owned by this SymbolVendor

```
dbg.evt-handler  Updating Symbol Scope for Stack ID with PC: 0x81e2062 CFA 0x7fa60c5f8ef0to Scope at addr 0x7fede17ca6f0, with Context: , Module{0x564277cfdf60}, CompileUnit{0x000002b0}, Function{0x7fffff00090bd34f}, Block{0x7fffff00090bd41d}
```

Then because the Block has been freed, we would (sometimes!) SEGV when calculating a backtrace.

Test Plan:
Load the HHVM Core that was causing the SEGV with the new build

```
(lldb) target create --core /tmp/cores/mp726nmh61xkbn9r
(lldb) auto-load-debuginfo
... (some omitted waiting)
(lldb) target list
Current targets:
* target #0: /var/tmp/cores/symbol_cache/fbpkg/hphp.admin.compiler_trunk_vm_clang_x86_64_profiled-49de0df41ef10b7389ae62da6ad27bd61702857f/x86_64/buck-out/opt-clang-hhvm-lto/gen/hphp/hhvm/hhvm/hhvm ( arch=x86_64-*-linux, platform=host, pid=2556, state=stopped )
(lldb) bt
* thread llvm#1, name = 'hhvm', stop reason = SIGSEGV: sent by tkill system call (sender pid=2556, uid=48)
  * frame #0: 0x00000000081e2062 hhvm`std::__shared_count<(__gnu_cxx::_Lock_policy)2>::__shared_count(this=<unavailable>, __r=0x0000000000000038, (null)=<unavailable>) at shared_ptr_base.h:937
    frame llvm#1: 0x00000000081e2052 hhvm`std::__shared_ptr<facebook::memcache::mcrouter::CarbonRouterInstance<facebook::tao::proto::TaoProtocolRouterInfo>, (__gnu_cxx::_Lock_policy)2>::__shared_ptr(this=0x00007fa60c5f8e30, __r=0x0000000000000030, (null)=<unavailable>) at shared_ptr_base.h:1409
truncated for brevity
```

Rollback Plan:

Reviewers: alexandreperez, satyajanga, gclayton, royshi, selinakim

Reviewed By: alexandreperez, satyajanga

Subscribers: generatedunixname499725568, #hhvm, #lldb_team

Differential Revision: https://phabricator.intern.facebook.com/D79481403

(cherry picked from commit 9ece231250df4980409c69c0c5905d7544290fa5)

Author: Jlalond

lldb/source/Core/Module.cpp

@@ -1223,6 +1223,11 @@ void Module::ReplaceObjectFile(Target &target, FileSpec object_file,
   m_object_offset = object_offset;
   lldb::addr_t load_address =
       GetObjectFile()->GetBaseAddress().GetLoadAddress(&target);
+  if (m_symfile_up) {
+    // Keep all old symbol files around in case there are any lingering type
+    // references in any SBValue objects that might have been handed out.
+    m_old_symfiles.push_back(std::move(m_symfile_up));
+  }
 
   // Scope locking.
   {