Minor interface cleanups of libgccjit
        * jit/jit-playback.c (convert_to_dso): Use auto_vec instead
        of automatic array to build up command line.
        * jit/jit-recording.c (recording::context::set_str_option):
        Make copy of the string.
        (recording::context::~context): Free string options.
        * jit/jit-recording.h (recording::context): Adjust type
        of m_str_options member.
        * jit/libgccjit.h: Adjust comment about
        gcc_jit_context_set_str_option parameter begin used after
        the call.
        Update comment now that all interfaces are copy strings
        if necessary.
        * jit/libgccjit++.h (gccjit::context): Add set_str_option
        member function.

From-SVN: r218617