Move dwarf2_cu to new header file

This moves dwarf2_cu and one supporting data structure to a new header
file.  The main goal, as always with this kind of change, is to make
the DWARF reader a bit more understandable.

gdb/ChangeLog
2021-05-17  Tom Tromey  <tom@tromey.com>

	* Makefile.in (HFILES_NO_SRCDIR): Add dwarf2/cu.h.
	* dwarf2/read.c (struct delayed_method_info, struct dwarf2_cu):
	Move to cu.h.
	* dwarf2/cu.h: New file.