Move some code from dwarf2/read.c to die.c

This patch introduces a new file, dwarf2/die.c, and moves some
DIE-related code out of dwarf2/read.c and into this new file.  This is
just a small part of the long-term project to split up read.c.
(According to 'wc', dwarf2/read.c is the largest file in gdb by around
8000 LOC.)

Regression tested on x86-64 Fedora 36.