Create new file dwarf2/sect-names.h

Create new file dwarf2/sect-names.h

This creates a new file, dwarf2/sect-names.h, and moves some
DWARF-specific type definitions from symfile.h into it.

gdb/ChangeLog
2021-03-06  Tom Tromey  <tom@tromey.com>

	* xcoffread.c: Include sect-names.h.
	* symfile.h (struct dwarf2_section_names, struct
	dwarf2_debug_sections): Move to dwarf2/sect-names.h.
	* dwarf2/sect-names.h: New file, from symfile.h.
	* dwarf2/read.c: Include sect-names.h.