Remove the objfile backlink from comp_unit
    
    This removes the objfile backlink from comp_unit.  The only remaining
    uses involved fetching the text offset from the objfile.  However,
    this is already conveniently computed at all the sites that call
    execute_cfa_program, and so it can simply be passed in.
    
    gdb/ChangeLog
    2020-02-12  Tom Tromey  <tom@tromey.com>
    
    	* dwarf2/frame.c (struct comp_unit) <objfile>: Remove.
    	(comp_unit): Don't initialize objfile.
    	(execute_cfa_program): Add text_offset parameter.
    	(execute_cfa_program_test, dwarf2_fetch_cfa_info)
    	(dwarf2_frame_cache): Update.
    	(dwarf2_build_frame_info): Don't set "objfile" member.