Move everything related to reading .debug_names from read.c to
read-debug-names.c. The only entry point exposed by
read-debug-names.{c,h} is dwarf2_read_debug_names.
Change-Id: I18b23f3c7a61b14abc3a46e4bf559bc2d078e8bc
Approved-By: Tom Tromey <tom@tromey.com>