[lldb] Fix crash on SBCommandReturnObject & assignment

I was writing an SB API client and it was crashing on:
	bool DoExecute(SBDebugger dbg, char **command, SBCommandReturnObject &result) {
	  result = subcommand(dbg, "help");

That is because SBCommandReturnObject &result gets initialized inside LLDB by:
	bool DoExecute(Args &command, CommandReturnObject &result) override {
	  // std::unique_ptr gets initialized here from `&result`!!!
	  SBCommandReturnObject sb_return(&result);
	  DoExecute(...);
	  sb_return.Release();

Differential revision: https://reviews.llvm.org/D67589

llvm-svn: 373775