Speedup access to unreferenced symbols when loading corpus_group

Speedup access to unreferenced symbols when loading corpus_group

This patch speeds up the invocation of
corpus_group::get_unreferenced_function_symbols and
corpus_group::get_unreferenced_variable_symbols.  It does so by
caching the result of constructing the map of of unreferenced symbol
at the time where each corpus is added to the group.

	* src/abg-corpus.cc (corpus_group::unrefed_{fun, var}_symbol_map):
	New data members.
	(corpus_group::priv::priv): Adjust.
	(corpus_group::priv::add_unref_{fun,var}_symbols): Define new
	member functions.
	(corpus_group::add_corpus): Update the map of unreferenced
	symbols.
	(corpus_group::get_unreferenced_{function,variable}_symbols)
	Adjust logic.

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