Fix broken assert

I was checking the wrong function before calling it.  Whoops!  Note
that assert() wasn't actually generating code because libclang exports
-DNDEBUG as a cflag -- this is terrible!
Filter -DNDEBUG from llvm-config

For some reason, -DNDEBUG was being defined by llvm-config.  This
caused my assert() calls to get compiled away.  I'm not sure why this
is being defined, but it certainly seems like a terrible idea to me --
just because I don't have assert() enabled in libclang doesn't mean I
want to disable assert() for every block of code I depend on!

This should fix the trouble I've been having when compiling on
different machines -- it turns out all of those didn't have LLVM
installed and therefor were building my internal version of it and not
defining -DNDEBUG.