Rename dwarf2/comp-unit.h

Rename dwarf2/comp-unit.h

Simon pointed out that dwarf2/cu.h and dwarf2/comp-unit.h seemingly
mean the same thing.  He suggested renaming the latter to
comp-unit-head.h, which is what this patch does.

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

	* dwarf2/read.h: Update include.
	* dwarf2/read.c: Update include.
	* dwarf2/line-header.c: Update include.
	* dwarf2/cu.h: Update include.
	* dwarf2/comp-unit-head.h: Rename from comp-unit.h.
	* dwarf2/comp-unit-head.c: Rename from comp-unit.c.
	* Makefile.in (COMMON_SFILES): Update.