c++: speed up looking up the current class

While looking at template instantiation tracing, I noticed that we were
frequently looking up a particular class template instance while
instantiating it.  This patch shortcuts that lookup, and speeds up compiling
stdc++.h with my (checking/unoptimized) compiler by about 3%.

gcc/cp/ChangeLog:

	* pt.c (lookup_template_class_1): Shortcut current_class_type.