Remove the objfile backlink from comp_unit

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.