Intrinsics for N2965: Type traits and base classes

[Benjamin Kosnik]

This is looking pretty good, from the libstdc++ side. This latest round
of gcc hacking fixes the previous testsuite fixes, so once you get
the gcc bits OK'd by Jason you can check in.

> I'd still like to see some testcases for the intrinsic, independent
> of the library.

Seems like some simple test case along the lines of 

gcc/testsuite/g++.dg/ext/is_base_of.C

will suffice. 

-benjamin