Rename some functions, index -> gdb_index

Rename some functions, index -> gdb_index

Since we now have two index formats, DWARF5/debug_names and gdb_index, I
wanted to rename some functions to make it clear that they deal with the
gdb_index format specifically.

gdb/ChangeLog:

	* dwarf2read.c (read_index_from_section): Rename to...
	(read_gdb_index_from_section): ... this, update all callers.
	(dwarf2_read_index): Rename to...
	(dwarf2_read_gdb_index): ... this, update all callers.