Rename struct attribute accessors

Rename struct attribute accessors

This removes the "value_" prefix from the struct value accessors.
This seemed unnecessarily wordy to me.

gdb/ChangeLog
2020-09-29  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (dwarf2_find_base_address, read_call_site_scope)
	(dwarf2_get_pc_bounds, dwarf2_record_block_ranges)
	(partial_die_info::read, dwarf2_string_attr, new_symbol): Update.
	* dwarf2/attribute.h (struct attribute): Rename methods.
	* dwarf2/attribute.c (attribute::as_address): Rename from
	value_as_address.
	(attribute::as_string): Rename from value_as_string.