Speed up class_decl::find_base_class

Speed up class_decl::find_base_class

This is useful for big debug info data set.  Also, this function is
going to be used extensively in subsequent patch that fixes a base
class representation related issue.

	* src/abg-ir.cc (class_decl::priv::bases_map_): New data member.
	(class_decl::add_base_specifier): Add the new base specifier to
	the new class_decl::priv::bases_map_ data member.
	(class_decl::find_base_class): Use the new
	class_decl::priv::bases_map_ data member to speed up finding the
	base class.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>