Tidy build_enum_type state variables.

Tidy build_enum_type state variables.

This patch brings the enum code closer to the class/union code, in the
hope that this will ease future code maintenance.

There are no behavioural changes.

	* src/abg-dwarf-reader.cc (build_enum_type): Rename local
	variable enum_is_anonymous to is_anonymous. Move initilisation
	of local variable is_artificial to location corresponding to
	that in add_or_update_class_type and add_or_update_union_type
	functions.

Signed-off-by: Giuliano Procida <gprocida@google.com>