Added a new host function that allows us to run shell command and get the...

Added a new host function that allows us to run shell command and get the output from them along with the status and signal:

Error
Host::RunShellCommand (const char *command,
                       const char *working_dir,
                       int *status_ptr,
                       int *signo_ptr,
                       std::string *command_output_ptr,
                       uint32_t timeout_sec);

This will allow us to use this functionality in the host lldb_private::Platform, and also use it in our lldb-platform binary. It leverages the existing code in Host::LaunchProcess and ProcessLaunchInfo.

llvm-svn: 154730