Fix isDerivedFrom matcher.

Without this patch, the isDerivedFrom matcher asserts in the
"assert(ClassDecl != NULL);" in the new test, as a
DependentTemplateSpecilizationType is not a sub-type of
TemplateSpecializationType and also does not offer getAsCXXRecordDecl().

I am not sure why this did not cause problems before. It is now (after
the changed implementation of isDerivedFrom) easier to write a matcher
that actually gets into this branch of the code.

llvm-svn: 164127