Change how accessibility is handled in dwarf2/read.c

Change how accessibility is handled in dwarf2/read.c

dwarf2/read.c uses dwarf2_default_access_attribute to check for the
default access attribute.  This patch simplifies the code by moving
more of the access processing into this function, changing its name to
reflect the difference.  This also ensures that the attribute's form
is respected, by changing to code to use the constant_value method.

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

	* dwarf2/read.c (dwarf2_access_attribute): Rename from
	dwarf2_default_access_attribute.  Look up attribute.
	(dwarf2_add_field, dwarf2_add_type_defn, dwarf2_add_member_fn):
	Update.