Introduce dwarf2/public.h

Introduce dwarf2/public.h

This moves some more DWARF code out of symfile.h and into a new
header, dwarf2/public.h.  This header is intended to describe the
public API of the DWARF reader.

gdb/ChangeLog
2021-03-20  Tom Tromey  <tom@tromey.com>

	* coffread.c: Include dwarf2/public.h.
	* dwarf2/frame.c: Include dwarf2/public.h.
	* dwarf2/index-write.h: Include dwarf2/public.h, not symfile.h.
	* dwarf2/public.h: New file.
	* dwarf2/read.c: Include dwarf2/public.h.
	* elfread.c: Include dwarf2/public.h.
	* machoread.c: Include dwarf2/public.h.
	* symfile.h (dwarf2_has_info, enum dw_index_kind)
	(dwarf2_initialize_objfile, dwarf2_build_psymtabs)
	(dwarf2_build_frame_info): Move to dwarf2/public.h.
	* xcoffread.c: Include dwarf2/public.h.