Change the behavior of the isDerivedFrom-matcher to not match on the

class itself. This caused some confusion (intuitively, a class is not
derived from itself) and makes it hard to write certain matchers, e.g.
"match and bind any pair of base and subclass".

The original behavior can be achieved with a new isA-matcher.  Similar
to all other matchers, this matcher has the same behavior and name as
the corresponding AST-entity - in this case the isa<>() function.

llvm-svn: 163385